fix prometheus test

Author: jack-berg

exporters/prometheus/src/test/java/io/opentelemetry/exporter/prometheus/PrometheusMetricReaderTest.java

@@ -972,29 +972,23 @@ void createdTimestamp() throws IOException {
 
     LongCounter counter = meter.counterBuilder("requests").build();
     testClock.advance(Duration.ofMillis(1));
+    long bearStartNanos = testClock.now();
     counter.add(3, Attributes.builder().put("animal", "bear").build());
     testClock.advance(Duration.ofMillis(1));
+    long mouseStartNanos = testClock.now();
     counter.add(2, Attributes.builder().put("animal", "mouse").build());
     testClock.advance(Duration.ofMillis(1));
 
-    // There is a curious difference between Prometheus and OpenTelemetry:
-    // In Prometheus metrics the _created timestamp is per data point,
-    // i.e. the _created timestamp says when this specific set of label values
-    // was first observed.
-    // In the OTel Java SDK the _created timestamp is the initialization time
-    // of the SdkMeterProvider, i.e. all data points will have the same _created timestamp.
-    // So we expect the _created timestamp to be the start time of the application,
-    // not the timestamp when the counter or an individual data point was created.
     String expected =
         ""
             + "# TYPE requests counter\n"
             + "requests_total{animal=\"bear\",otel_scope_name=\"test\"} 3.0\n"
             + "requests_created{animal=\"bear\",otel_scope_name=\"test\"} "
-            + createdTimestamp
+            + convertTimestamp(bearStartNanos)
             + "\n"
             + "requests_total{animal=\"mouse\",otel_scope_name=\"test\"} 2.0\n"
             + "requests_created{animal=\"mouse\",otel_scope_name=\"test\"} "
-            + createdTimestamp
+            + convertTimestamp(mouseStartNanos)
             + "\n"
             + "# TYPE target info\n"
             + "target_info{service_name=\"unknown_service:java\",telemetry_sdk_language=\"java\",telemetry_sdk_name=\"opentelemetry\",telemetry_sdk_version=\"1.x.x\"} 1\n"