End Prometheus stats with a new line separator (postgresml#826)

End prometheus stats with a new line separator

According to the [OpenMetrics specification](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#overall-structure), each line MUST end with `\n`. Previously, the last line was not ending with `\n`, so that strict parsers had issues reading the Prometheus stats.

src/prometheus.rs

@@ -309,6 +309,7 @@ async fn prometheus_stats(
             push_pool_stats(&mut lines);
             push_server_stats(&mut lines);
             push_database_stats(&mut lines);
+            lines.push("".to_string()); // Ensure to end the stats with a line terminator as required by the specification.
 
             Response::builder()
                 .header("content-type", "text/plain; version=0.0.4")