Allow EventSample.record to accept numSamples=0

There's really no reason not to.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171037754

java/google/registry/monitoring/metrics/contrib/BUILD

@@ -10,8 +10,8 @@ java_library(
     srcs = glob(["*.java"]),
     deps = [
         "//java/google/registry/monitoring/metrics",
+        "//third_party/java/truth",
         "@com_google_code_findbugs_jsr305",
         "@com_google_guava",
-        "@com_google_truth",
     ],
 )