fix: add endpoint override flag (#3050)

* fix: add endpoint override flag

This change is needed because the parameter `endpoint` will always be populated even if a value was not provided at client construction, and
therefore we can not relay on it to determine its source.

* chore: fix endpoint override test

src/ClientResolver.php

@@ -1030,6 +1030,7 @@ public static function _apply_endpoint($value, array &$args, HandlerList $list)
             return;
         }
 
+        $args['endpoint_override'] = true;
         $args['endpoint'] = $value;
     }
 

src/UserAgentMiddleware.php

@@ -276,7 +276,7 @@ private function getMetrics(): string
      */
     private function appendEndpointMetric(): void
     {
-        if (!empty($this->args['endpoint'])) {
+        if (!empty($this->args['endpoint_override'])) {
             $this->metricsBuilder->append(MetricsBuilder::ENDPOINT_OVERRIDE);
         }
     }

tests/UserAgentMiddlewareTest.php

@@ -204,7 +204,8 @@ public function userAgentCasesDataProvider(): \Generator
             'metricsWithEndpoint' => function (): array {
                 $expectedEndpoint = "https://foo-endpoint.com";
                 $args = [
-                    'endpoint' => $expectedEndpoint
+                    'endpoint' => $expectedEndpoint,
+                    'endpoint_override' => true,
                 ];
 
                 return [$args, 'm/' . MetricsBuilder::ENDPOINT_OVERRIDE];