feat: Expose metric dictionary to make logging metrics as JSON easier

docs/implementation/logging.md

@@ -44,36 +44,41 @@ Users of a tap can configure the SDK logging by setting the `SINGER_SDK_LOG_CONF
 environment variable. The value of this variable should be a path to a YAML file in the
 [Python logging dict format](https://docs.python.org/3/library/logging.config.html#dictionary-schema-details).
 
-For example, to send [metrics](./metrics.md) (with logger name `singer_sdk.metrics`) to a file, you could use the following config:
+### Metrics logging
+
+The Singer SDK provides a logger named `singer_sdk.metrics` for logging [Singer metrics](./metrics.md). Metric log records contain an extra field `point` which is a dictionary containing the metric data. The `point` field is formatted as JSON by default.
+
+To send metrics to a file in JSON format, you could use the following config:
 
 ```yaml
 version: 1
 disable_existing_loggers: false
 formatters:
   metrics:
-    format: "{asctime} {levelname} {message}"
+    (): pythonjsonlogger.jsonlogger.JsonFormatter
+    format: "{created} {message} {point}"
     style: "{"
 handlers:
   metrics:
     class: logging.FileHandler
     formatter: metrics
-    filename: metrics.log
+    filename: metrics.jsonl
 loggers:
   singer_sdk.metrics:
     level: INFO
     handlers: [ metrics ]
-    propagate: yes
+    propagate: no
 ```
 
-This will send metrics to a `metrics.log`:
+This will send metrics to a `metrics.jsonl`:
 
-```
-2022-09-29 00:48:52,746 INFO METRIC: {"metric_type": "timer", "metric": "http_request_duration", "value": 0.501743, "tags": {"stream": "continents", "endpoint": "", "http_status_code": 200, "status": "succeeded"}}
-2022-09-29 00:48:52,775 INFO METRIC: {"metric_type": "counter", "metric": "http_request_count", "value": 1, "tags": {"stream": "continents", "endpoint": ""}}
-2022-09-29 00:48:52,776 INFO METRIC: {"metric_type": "timer", "metric": "sync_duration", "value": 0.7397160530090332, "tags": {"stream": "continents", "context": {}, "status": "succeeded"}}
-2022-09-29 00:48:52,776 INFO METRIC: {"metric_type": "counter", "metric": "record_count", "value": 7, "tags": {"stream": "continents", "context": {}}}
-2022-09-29 00:48:53,225 INFO METRIC: {"metric_type": "timer", "metric": "http_request_duration", "value": 0.392148, "tags": {"stream": "countries", "endpoint": "", "http_status_code": 200, "status": "succeeded"}}
-2022-09-29 00:48:53,302 INFO METRIC: {"metric_type": "counter", "metric": "http_request_count", "value": 1, "tags": {"stream": "countries", "endpoint": ""}}
-2022-09-29 00:48:53,302 INFO METRIC: {"metric_type": "timer", "metric": "sync_duration", "value": 0.5258760452270508, "tags": {"stream": "countries", "context": {}, "status": "succeeded"}}
-2022-09-29 00:48:53,303 INFO METRIC: {"metric_type": "counter", "metric": "record_count", "value": 250, "tags": {"stream": "countries", "context": {}}}
+```json
+{"created": 1705709074.883021, "message": "METRIC", "point": {"type": "timer", "metric": "http_request_duration", "value": 0.501743, "tags": {"stream": "continents", "endpoint": "", "http_status_code": 200, "status": "succeeded"}}}
+{"created": 1705709074.897184, "message": "METRIC", "point": {"type": "counter", "metric": "http_request_count", "value": 1, "tags": {"stream": "continents", "endpoint": ""}}}
+{"created": 1705709074.897256, "message": "METRIC", "point": {"type": "timer", "metric": "sync_duration", "value": 0.7397160530090332, "tags": {"stream": "continents", "context": {}, "status": "succeeded"}}}
+{"created": 1705709074.897292, "message": "METRIC", "point": {"type": "counter", "metric": "record_count", "value": 7, "tags": {"stream": "continents", "context": {}}}}
+{"created": 1705709075.397254, "message": "METRIC", "point": {"type": "timer", "metric": "http_request_duration", "value": 0.392148, "tags": {"stream": "countries", "endpoint": "", "http_status_code": 200, "status": "succeeded"}}}
+{"created": 1705709075.421888, "message": "METRIC", "point": {"type": "counter", "metric": "http_request_count", "value": 1, "tags": {"stream": "countries", "endpoint": ""}}}
+{"created": 1705709075.422001, "message": "METRIC", "point": {"type": "timer", "metric": "sync_duration", "value": 0.5258760452270508, "tags": {"stream": "countries", "context": {}, "status": "succeeded"}}}
+{"created": 1705709075.422047, "message": "METRIC", "point": {"type": "counter", "metric": "record_count", "value": 250, "tags": {"stream": "countries", "context": {}}}}
 ```

pyproject.toml

@@ -55,6 +55,7 @@ pendulum = ">=2.1.0,<4"
 PyJWT = "~=2.4"
 python-dateutil = ">=2.8.2"
 python-dotenv = ">=0.20"
+python-json-logger = ">=2"
 PyYAML = ">=6.0"
 referencing = ">=0.30.0"
 requests = ">=2.25.1"

singer_sdk/default_logging.yml

@@ -4,11 +4,24 @@ formatters:
   console:
     format: "{asctime:23s} | {levelname:8s} | {name:20s} | {message}"
     style: "{"
+  metrics:
+    (): singer_sdk.metrics.SingerMetricsFormatter
+    format: "{asctime:23s} | {levelname:8s} | {name:20s} | {message}: {metric_json}"
+    style: "{"
 handlers:
   default:
     class: logging.StreamHandler
     formatter: console
     stream: ext://sys.stderr
+  metrics:
+    class: logging.StreamHandler
+    formatter: metrics
+    stream: ext://sys.stderr
 root:
   level: INFO
   handlers: [default]
+loggers:
+  singer_sdk.metrics:
+    level: INFO
+    handlers: [ metrics ]
+    propagate: no

singer_sdk/metrics.py

@@ -67,29 +67,35 @@ class Point(t.Generic[_TVal]):
     value: _TVal
     tags: dict[str, t.Any] = field(default_factory=dict)
 
-    def __str__(self) -> str:
-        """Get string representation of this measurement.
+    def to_dict(self) -> dict[str, t.Any]:
+        """Convert this measure to a dictionary.
 
         Returns:
-            A string representation of this measurement.
+            A dictionary.
         """
-        return self.to_json()
+        return {
+            "type": self.metric_type,
+            "metric": self.metric.value,
+            "value": self.value,
+            "tags": self.tags,
+        }
 
-    def to_json(self) -> str:
-        """Convert this measure to a JSON object.
+
+class SingerMetricsFormatter(logging.Formatter):
+    """Logging formatter that adds a ``metric_json`` field to the log record."""
+
+    def format(self, record: logging.LogRecord) -> str:
+        """Format a log record.
+
+        Args:
+            record: A log record.
 
         Returns:
-            A JSON object.
+            A formatted log record.
         """
-        return json.dumps(
-            {
-                "type": self.metric_type,
-                "metric": self.metric.value,
-                "value": self.value,
-                "tags": self.tags,
-            },
-            default=str,
-        )
+        point = record.__dict__.get("point")
+        record.__dict__["metric_json"] = json.dumps(point, default=str) if point else ""
+        return super().format(record)
 
 
 def log(logger: logging.Logger, point: Point) -> None:
@@ -99,7 +105,7 @@ def log(logger: logging.Logger, point: Point) -> None:
         logger: An logger instance.
         point: A measurement.
     """
-    logger.info("METRIC: %s", point)
+    logger.info("METRIC", extra={"point": point.to_dict()})
 
 
 class Meter(metaclass=abc.ABCMeta):

tests/core/test_metrics.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import json
 import logging
 
 import pytest
@@ -17,6 +18,32 @@ def __str__(self) -> str:
         return f"{self.name}={self.value}"
 
 
+def test_singer_metrics_formatter():
+    formatter = metrics.SingerMetricsFormatter(fmt="{metric_json}", style="{")
+
+    record = logging.LogRecord(
+        name="test",
+        level=logging.INFO,
+        pathname="test.py",
+        lineno=1,
+        msg="METRIC",
+        args=(),
+        exc_info=None,
+    )
+
+    assert formatter.format(record) == ""
+
+    metric_dict = {
+        "type": "counter",
+        "metric": "test_metric",
+        "tags": {"test_tag": "test_value"},
+        "value": 1,
+    }
+    record.__dict__["point"] = metric_dict
+
+    assert formatter.format(record) == json.dumps(metric_dict)
+
+
 def test_meter():
     class _MyMeter(metrics.Meter):
         def __enter__(self):
@@ -38,6 +65,9 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
 
 def test_record_counter(caplog: pytest.LogCaptureFixture):
+    metrics_logger = logging.getLogger(metrics.METRICS_LOGGER_NAME)
+    metrics_logger.propagate = True
+
     caplog.set_level(logging.INFO, logger=metrics.METRICS_LOGGER_NAME)
     custom_object = CustomObject("test", 1)
 
@@ -56,29 +86,33 @@ def test_record_counter(caplog: pytest.LogCaptureFixture):
     total = 0
 
     assert len(caplog.records) == 100 + 1
+    # raise AssertionError
 
     for record in caplog.records:
         assert record.levelname == "INFO"
-        assert record.msg == "METRIC: %s"
-        assert "test=1" in record.message
+        assert record.msg.startswith("METRIC")
 
-        point: metrics.Point[int] = record.args[0]
-        assert point.metric_type == "counter"
-        assert point.metric == "record_count"
-        assert point.tags == {
+        point = record.__dict__["point"]
+        assert point["type"] == "counter"
+        assert point["metric"] == "record_count"
+        assert point["tags"] == {
             metrics.Tag.STREAM: "test_stream",
             metrics.Tag.ENDPOINT: "test_endpoint",
             "custom_tag": "pytest",
             "custom_obj": custom_object,
         }
 
-        total += point.value
+        total += point["value"]
 
     assert total == 100
 
 
 def test_sync_timer(caplog: pytest.LogCaptureFixture):
+    metrics_logger = logging.getLogger(metrics.METRICS_LOGGER_NAME)
+    metrics_logger.propagate = True
+
     caplog.set_level(logging.INFO, logger=metrics.METRICS_LOGGER_NAME)
+
     traveler = time_machine.travel(0, tick=False)
     traveler.start()
 
@@ -92,15 +126,15 @@ def test_sync_timer(caplog: pytest.LogCaptureFixture):
 
     record = caplog.records[0]
     assert record.levelname == "INFO"
-    assert record.msg == "METRIC: %s"
+    assert record.msg.startswith("METRIC")
 
-    point: metrics.Point[float] = record.args[0]
-    assert point.metric_type == "timer"
-    assert point.metric == "sync_duration"
-    assert point.tags == {
+    point = record.__dict__["point"]
+    assert point["type"] == "timer"
+    assert point["metric"] == "sync_duration"
+    assert point["tags"] == {
         metrics.Tag.STREAM: "test_stream",
         metrics.Tag.STATUS: "succeeded",
         "custom_tag": "pytest",
     }
 
-    assert pytest.approx(point.value, rel=0.001) == 10.0
+    assert pytest.approx(point["value"], rel=0.001) == 10.0