fix: fix typing issue with gRPC metadata when key ends in -bin (#2251)

Author: parthea

Diff for: gapic/samplegen/samplegen.py

@@ -1107,7 +1107,7 @@ def _fill_sample_metadata(sample: dict, api_schema: api.API):
     parameters.append(snippet_metadata_pb2.ClientMethod.Parameter(  # type: ignore
         name="timeout", type="float"))
     parameters.append(snippet_metadata_pb2.ClientMethod.Parameter(  # type: ignore
-        name="metadata", type="Sequence[Tuple[str, str]"))
+        name="metadata", type="Sequence[Tuple[str, Union[str, bytes]]]"))
 
     return snippet_metadata
 

Diff for: gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2

@@ -454,9 +454,11 @@ class _{{ name }}(_Base{{ service.name }}RestTransport._Base{{name}}, {{ async_m
 
 {% macro client_method_metadata_argument() %}metadata: {{ client_method_metadata_type() }}{% endmacro %}
 
-{% macro client_method_metadata_type() %}Sequence[Tuple[str, str]]{% endmacro %}
+{% macro client_method_metadata_type() %}Sequence[Tuple[str, Union[str, bytes]]]{% endmacro %}
 
 {% macro client_method_metadata_default_value() %}(){% endmacro %}
 
-{% macro client_method_metadata_argument_doc() %}metadata ({{ client_method_metadata_type() }}): Strings which should be
-            sent along with the request as metadata.{% endmacro %}
+{% macro client_method_metadata_argument_doc() %}metadata ({{ client_method_metadata_type() }}): Key/value pairs which should be 
+            sent along with the request as metadata. Normally, each value must be of type `str`,
+            but for metadata keys ending with the suffix `-bin`, the corresponding values must
+            be of type `bytes`.{% endmacro %}