fix(otel): Monitored Resource with task_id for Cloud Run

Fixes googleapis#14925

When inside a Cloud Run environment, the `MonitoredResource` in a `CreateTimeSeriesRequest` to the Cloud Monitoring API does not include the necessary fields for the `generic_task` resource type, and is rejected.

Should follow the well-tested Golang implementation where the `faas.instance` OTel Resource Attribute is mapped to `MonitoredResource` `task_id`. As the `service.namespace` OTel Resource Attribute is not set by the Resource Detector from within Cloud Run, it should be mapped as an empty string, rather than being left absent.

https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/blob/8da0f42dab085c916987891419461d583a2aa96e/internal/resourcemapping/resourcemapping.go#L153

google/cloud/opentelemetry/internal/monitored_resource.cc

@@ -74,6 +74,8 @@ class MonitoredResourceProvider {
         mr.labels[kv.first] = AsString(attributes.at(*found));
       } else if (kv.second.fallback) {
         mr.labels[kv.first] = *kv.second.fallback;
+      } else {
+        mr.labels[kv.first] = "";
       }
     }
     return mr;
@@ -162,8 +164,8 @@ MonitoredResourceProvider GenericTask() {
           {"location",
            {{sc::kCloudAvailabilityZone, sc::kCloudRegion}, "global"}},
           {"namespace", {{sc::kServiceNamespace}}},
-          {"job", {{sc::kServiceName}}},
-          {"task_id", {{sc::kServiceInstanceId}}},
+          {"job", {{sc::kServiceName, sc::kFaasName}}},
+          {"task_id", {{sc::kServiceInstanceId, sc::kFaasInstance}}},
       });
 }
 
@@ -174,7 +176,7 @@ MonitoredResourceProvider GenericNode() {
           {"location",
            {{sc::kCloudAvailabilityZone, sc::kCloudRegion}, "global"}},
           {"namespace", {{sc::kServiceNamespace}}},
-          {"node_id", {{sc::kHostId}}},
+          {"node_id", {{sc::kHostId, sc::kHostName}}},
       });
 }