chore: add a couple of tests for the new metrics impl

Change-Id: If047feb0b01e56bc3acf8be8c976e9c8e5d16b6b

Author: igorbernstein2

google-cloud-bigtable/pom.xml

@@ -255,6 +255,11 @@
     </dependency>
 
     <!-- Test dependencies -->
+    <dependency>
+      <groupId>com.google.api.grpc</groupId>
+      <artifactId>grpc-google-cloud-monitoring-v3</artifactId>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>com.google.api</groupId>
       <artifactId>gax</artifactId>