feat: adding gcp resource name span attribute (#2260)

Adding a new span attribute called `gcp.resource.name` which contains an identifier to a particular firestore database in the following format:
```
//firestore.googleapis.com/projects/{project}/databases/{database_id}
```

Example:
```
//firestore.googleapis.com/projects/my_project/databases/(default)
```

Author: Gautam04

google-cloud-firestore/src/main/java/com/google/cloud/firestore/telemetry/EnabledTraceUtil.java

@@ -283,6 +283,12 @@ private SpanBuilder addSettingsAttributesToCurrentSpan(SpanBuilder spanBuilder)
     spanBuilder =
         spanBuilder.setAllAttributes(
             Attributes.builder()
+                .put(
+                    ATTRIBUTE_GCP_RESOURCE_NAME,
+                    String.format(
+                        ATTRIBUTE_GCP_RESOURCE_NAME_FORMAT,
+                        firestoreOptions.getProjectId(),
+                        firestoreOptions.getDatabaseId()))
                 .put(
                     ATTRIBUTE_SERVICE_PREFIX + "settings.project_id",
                     firestoreOptions.getProjectId())

google-cloud-firestore/src/main/java/com/google/cloud/firestore/telemetry/TraceUtil.java

@@ -40,6 +40,8 @@ public interface TraceUtil {
   String ATTRIBUTE_KEY_TRANSACTION_TYPE = "transaction_type";
   String ATTRIBUTE_KEY_ATTEMPTS_ALLOWED = "attempts_allowed";
   String ATTRIBUTE_KEY_ATTEMPTS_REMAINING = "attempts_remaining";
+  String ATTRIBUTE_GCP_RESOURCE_NAME = "gcp.resource.name";
+  String ATTRIBUTE_GCP_RESOURCE_NAME_FORMAT = "//firestore.googleapis.com/projects/%s/databases/%s";
 
   String ENABLE_TRACING_ENV_VAR = "FIRESTORE_ENABLE_TRACING";
   String LIBRARY_NAME = "com.google.cloud.firestore";

google-cloud-firestore/src/test/java/com/google/cloud/firestore/it/ITTracingTest.java

@@ -281,6 +281,7 @@ void assertHasExpectedAttributes(SpanData spanData, String... additionalExpected
     // All Firestore-generated spans have the settings attributes.
     List<String> expectedAttributes =
         Arrays.asList(
+            "gcp.resource.name",
             "gcp.firestore.memory_utilization",
             "gcp.firestore.settings.host",
             "gcp.firestore.settings.project_id",