bigtable: RowMutation should allow passing of a Mutation (googleapis#3643)

Fixes googleapis#3637

Author: ajaaym

google-cloud-clients/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/models/RowMutation.java

@@ -37,18 +37,54 @@ public final class RowMutation implements MutationApi<RowMutation>, Serializable
   private final ByteString key;
   private final Mutation mutation;
 
-  private RowMutation(String tableId, ByteString key) {
+  private RowMutation(String tableId, ByteString key, Mutation mutation) {
     this.tableId = tableId;
     this.key = key;
-    this.mutation = Mutation.create();
+    this.mutation = mutation;
   }
 
+  /** Creates a new instance of the mutation builder. */
   public static RowMutation create(@Nonnull String tableId, @Nonnull String key) {
     return create(tableId, ByteString.copyFromUtf8(key));
   }
 
+  /** Creates a new instance of the mutation builder. */
   public static RowMutation create(@Nonnull String tableId, @Nonnull ByteString key) {
-    return new RowMutation(tableId, key);
+    return new RowMutation(tableId, key, Mutation.create());
+  }
+
+  /**
+   * Creates new instance of mutation builder by wrapping existing set of row mutations.
+   * The builder will be owned by this RowMutation and should not be used by the caller after this call.
+   * This functionality is intended for advanced usage.
+   *
+   * <p>Sample code:
+   *
+   * <pre><code>
+   * Mutation mutation = Mutation.create()
+   *     .setCell("[FAMILY_NAME]", "[QUALIFIER]", [TIMESTAMP], "[VALUE]");
+   * RowMutation rowMutation = RowMutation.create("[TABLE]", "[ROW_KEY]", mutation);
+   * </code></pre>
+   */
+  public static RowMutation create(@Nonnull String tableId, @Nonnull String key, @Nonnull Mutation mutation) {
+    return create(tableId, ByteString.copyFromUtf8(key), mutation);
+  }
+
+  /**
+   * Creates new instance of mutation builder by wrapping existing set of row mutations.
+   * The builder will be owned by this RowMutation and should not be used by the caller after this call.
+   * This functionality is intended for advanced usage.
+   *
+   * <p>Sample code:
+   *
+   * <pre><code>
+   * Mutation mutation = Mutation.create()
+   *     .setCell("[FAMILY_NAME]", "[QUALIFIER]", [TIMESTAMP], "[VALUE]");
+   * RowMutation rowMutation = RowMutation.create("[TABLE]", [BYTE_STRING_ROW_KEY], mutation);
+   * </code></pre>
+   */
+  public static RowMutation create(@Nonnull String tableId, @Nonnull ByteString key, @Nonnull Mutation mutation) {
+    return new RowMutation(tableId, key, mutation);
   }
 
   @Override

google-cloud-clients/google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/models/RowMutationTest.java

@@ -90,6 +90,16 @@ public void toBulkProtoTest() {
         .isIn(timestampRange);
   }
 
+  @Test
+  public void toProtoTestWithProvidedMutation() {
+    Mutation mutation = Mutation.create().setCell("fake-family", "fake-qualifier", "fake-value");
+    RowMutation rowMutation = RowMutation.create("fake-table", "fake-key", mutation);
+
+    MutateRowRequest actualRowMutation = rowMutation.toProto(REQUEST_CONTEXT);
+
+    assertThat(actualRowMutation.getMutationsList()).isEqualTo(mutation.getMutations());
+  }
+
   @Test
   public void serializationTest() throws IOException, ClassNotFoundException {
     RowMutation expected =