Log prometheus listener error at Warn level (#7291)

## What changed?
<!-- Describe what has changed in this PR -->
- Log prometheus listener error at Warn level

## Why?
<!-- Tell your future self why have you made these changes -->
- For backward compatibility. There might be some CI systems monitoring
logs and fail tests on Error logs.

common/metrics/opentelemetry_provider.go

@@ -132,7 +132,9 @@ func initPrometheusListener(
 		if fatalOnListenerError {
 			logger.Fatal(msg)
 		} else {
-			logger.Error(msg)
+			// For backward compatibility, we log as Warn instead of Error/Fatal
+			// to match the behavior of tally framework.
+			logger.Warn(msg)
 		}
 	}()