[chore] prom rw exporter add extra logs for the RW2 code path

exporter/prometheusremotewriteexporter/exporter.go

@@ -411,6 +411,21 @@ func (prwe *prwExporter) execute(ctx context.Context, buf *buffer) error {
 			resp.Body.Close()
 		}()
 
+		// Per the Prometheus remote write 2.0 specification, the response should contain
+		// X-Prometheus-Remote-Write-Samples-Written header.
+		// If the header is missing, it suggests that the endpoint does not support RW2 or the
+		// implementation is not compliant with the specification. Reference:
+		// https://prometheus.io/docs/specs/prw/remote_write_spec_2_0/#required-written-response-headers
+		if enableSendingRW2FeatureGate.IsEnabled() && prwe.RemoteWriteProtoMsg == config.RemoteWriteProtoMsgV2 {
+			samplesWritten := resp.Header.Get("X-Prometheus-Remote-Write-Samples-Written")
+			if samplesWritten == "" {
+				prwe.settings.Logger.Warn(
+					"X-Prometheus-Remote-Write-Samples-Written header is missing from the response, suggesting that the endpoint doesn't support RW2 and might be silently dropping data.",
+					zap.String("url", resp.Request.URL.String()),
+				)
+			}
+		}
+
 		// 2xx status code is considered a success
 		// 5xx errors are recoverable and the exporter should retry
 		// Reference for different behavior according to status code: