feat: [Bigtable] add support for filtering SampleRowKeys to a specific row_range

Bigtable/samples/V2/BigtableClient/sample_row_keys.php

@@ -35,6 +35,9 @@
  * which can be used to break up the data for distributed tasks like
  * mapreduces.
  *
+ * If a `row_range` is provided in the request, the returned samples will be
+ * restricted to the specified range.
+ *
  * @param string $formattedTableName Optional. The unique name of the table from which to sample row keys.
  *
  *                                   Values are of the form

Bigtable/src/V2/Client/BigtableClient.php

@@ -691,6 +691,9 @@ public function readRows(ReadRowsRequest $request, array $callOptions = []): Ser
      * which can be used to break up the data for distributed tasks like
      * mapreduces.
      *
+     * If a `row_range` is provided in the request, the returned samples will be
+     * restricted to the specified range.
+     *
      * @example samples/V2/BigtableClient/sample_row_keys.php
      *
      * @param SampleRowKeysRequest $request     A request to house fields associated with the call.