add the file

google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/stub/MetadataSubject.java

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+package com.google.cloud.bigtable.data.v2.stub;
+
+import static com.google.common.truth.Truth.assertAbout;
+
+import com.google.common.truth.FailureMetadata;
+import com.google.common.truth.Subject;
+import io.grpc.Metadata;
+import javax.annotation.Nullable;
+
+/** Utility class to test key-value pairs in {@link io.grpc.Metadata}. */
+final class MetadataSubject extends Subject {
+
+  @Nullable private final Metadata actual;
+
+  public static Factory<MetadataSubject, Metadata> metadata() {
+    return MetadataSubject::new;
+  }
+
+  private MetadataSubject(FailureMetadata metadata, @Nullable Metadata actual) {
+    super(metadata, actual);
+    this.actual = actual;
+  }
+
+  public static MetadataSubject assertThat(@Nullable Metadata actual) {
+    return assertAbout(metadata()).that(actual);
+  }
+
+  public void containsAtLeast(String... keyValuePairs) {
+    assert actual != null;
+    for (int i = 0; i < keyValuePairs.length; i += 2) {
+      check("containsAtLeast()")
+          .that(actual.get(Metadata.Key.of(keyValuePairs[i], Metadata.ASCII_STRING_MARSHALLER)))
+          .isEqualTo(keyValuePairs[i + 1]);
+    }
+  }
+
+  public void doesNotContainKeys(String... keys) {
+    assert actual != null;
+    for (String key : keys) {
+      check("doesNotContainKeys()")
+          .that(actual.containsKey(Metadata.Key.of(key, Metadata.ASCII_STRING_MARSHALLER)))
+          .isFalse();
+    }
+  }
+}