Improve StackdriverWriter code

Add additional logic to handle cases when a MetricDescriptor is already defined
on the server, fix an NPE in related code, and add additional tests to ensure
that TimeSeries created from native MetricPoints are well-formed.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130555708

java/google/registry/monitoring/metrics/BUILD

@@ -10,7 +10,7 @@ java_library(
     srcs = glob(["*.java"]),
     deps = [
         "//google/monitoring:monitoring_java_lib",
-        "//java/com/google/api/client/json",
+        "//java/com/google/api/client/googleapis/json",
         "//java/com/google/common/annotations",
         "//java/com/google/common/base",
         "//java/com/google/common/collect",