Fix flaky protobuf debug format test when nanos is zero (#1958)

## Summary
- Protobuf3 text format omits fields with default (zero) values
- When `created_timestamp` lands on an exact second boundary, the
`nanos` field is absent
- The test pattern now makes the `nanos` line optional

## Test plan
- [ ] CI passes (specifically Java 25 integration tests)

---------

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

integration-tests/it-exporter/it-exporter-test/src/test/java/io/prometheus/metrics/it/exporter/test/ExporterIT.java

@@ -110,11 +110,15 @@ public void testPrometheusProtobufDebugFormat(String format, String expected) th
             .replace("<app>", sampleApp);
 
     if ("prometheus-protobuf".equals(format)) {
-      assertThat(actualResponse)
-          .matches(
-              Pattern.quote(expectedResponse)
-                  .replace("<CREATED_TIMESTAMP_SECONDS>", "\\E\\d+\\Q")
-                  .replace("<CREATED_TIMESTAMP_NANOS>", "\\E\\d+\\Q"));
+      // Protobuf text format omits fields with value 0, so nanos may be absent.
+      // Replace the nanos placeholder with a regex-friendly marker before quoting.
+      String withOptionalNanos =
+          expectedResponse.replace("\n      nanos: <CREATED_TIMESTAMP_NANOS>", "<OPTIONAL_NANOS>");
+      String pattern =
+          Pattern.quote(withOptionalNanos)
+              .replace("<CREATED_TIMESTAMP_SECONDS>", "\\E\\d+\\Q")
+              .replace("<OPTIONAL_NANOS>", "\\E(\n      nanos: \\d+)?\\Q");
+      assertThat(actualResponse).matches(pattern);
     } else {
       assertThat(actualResponse).isEqualTo(expectedResponse);
     }