Debug log for failed validation of alert rules needs to be decoded from bytes

[sed-i]

When rules validation via cos-tool fails, we get a debug log like this:

Validating the rules failed: b'error validating /tmp/... (very long output)'

In charm code, we return the errors decoded, but when we print to debug-log, we do not decode:

prometheus-k8s-operator/lib/charms/prometheus_k8s/v0/prometheus_scrape.py

Lines 2316 to 2324 in d36ffa5

	except subprocess.CalledProcessError as e:
	logger.debug("Validating the rules failed: %s", e.output)
	return False, ", ".join(
	[
	line
	for line in e.output.decode("utf8").splitlines()
	if "error validating" in line
	]
	)

Also, the debug log line includes golang references that are not useful here.

This is repeated also here:

prometheus-k8s-operator/lib/charms/grafana_k8s/v0/grafana_dashboard.py

Line 1967 in d36ffa5

logger.debug("Validating the rules failed: %s", e.output)

prometheus-k8s-operator/lib/charms/prometheus_k8s/v1/prometheus_remote_write.py

Line 917 in d36ffa5

logger.debug("Validating the rules failed: %s", e.output)

Summary

1. When validation fails, cos-tool shouldn't dump golang exception log.
2. When validation fails, we should print to debug-log the decoded error, not the bytes.
3. The CosTool class should be migrated to cos-lib, to remove duplication in prometheus_scrape, prometheus_remote_write, grafana_dashboard.