feat: Make grpc transcode logic work in terms of protobuf python obje…

…cts (#428)

* feat: Make grpc transcode logic work in terms of protobuf python objects

(for context: [gRPC Transcoding](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto#L44))
Previously it worked on dictionaries only, but that causes problems.

In GAPIC the dictionaries are created through the same logic as JSON (there is no better built-in way), thus applying [protobuf json mapping](https://developers.google.com/protocol-buffers/docs/proto3#json) conversion logic in the process. Unfortunately converting a protobuf object to a dictionary and to JSON, although similar, are not the same thing. Specifically the `Timestamp`, `Duration`, `FieldMask`, `uint64`, `int64`, and `*Value` protobuf messages are converted to strings for JSON (instead of being properly converted to dicts for most of those types, and `int64/uint64` converted to `int` respectively). As a result a rountrip that GAPIC was relying on (protobuf object -> dict -> transcode -> protobuf object) did not work properly. For example, when converted to dictionary, every int64 field would be converted to `string` (because it is what proto-JSON mapping spec requires), but later, when we need to rebuild a message from a transcoded dictionary that would fail with the following error:
```
TypeError: '0' has type str, but expected one of: int
```

Note, `*Rules` thing from proto-plus does not help, becuase the type may happen inside common native protobuf stub messsages (like `google.type.Money`), fields of which are outside of scope of the proto-plus custom conversion logic.

Also, this change greatly simplifies the procedure of transcodding, eliminating multiple conversion steps (to and from dictionaries multiple times) making the whole logic significanly more efficient (python gapics are nutoriously known to be slow due to proto-plus stuff, so efficiency is important) and robust (JSON conversion logic does not interfere anymore with pure protobuf objects grpc transcoding)

* reformat code using black

* reformat code according to flake8

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

google/api_core/path_template.py

@@ -176,18 +176,20 @@ def get_field(request, field):
     """Get the value of a field from a given dictionary.
 
     Args:
-        request (dict): A dictionary object.
+        request (dict | Message): A dictionary or a Message object.
         field (str): The key to the request in dot notation.
 
     Returns:
         The value of the field.
     """
     parts = field.split(".")
     value = request
+
     for part in parts:
         if not isinstance(value, dict):
-            return
-        value = value.get(part)
+            value = getattr(value, part, None)
+        else:
+            value = value.get(part)
     if isinstance(value, dict):
         return
     return value
@@ -197,19 +199,27 @@ def delete_field(request, field):
     """Delete the value of a field from a given dictionary.
 
     Args:
-        request (dict): A dictionary object.
+        request (dict | Message): A dictionary object or a Message.
         field (str): The key to the request in dot notation.
     """
     parts = deque(field.split("."))
     while len(parts) > 1:
-        if not isinstance(request, dict):
-            return
         part = parts.popleft()
-        request = request.get(part)
+        if not isinstance(request, dict):
+            if hasattr(request, part):
+                request = getattr(request, part, None)
+            else:
+                return
+        else:
+            request = request.get(part)
     part = parts.popleft()
     if not isinstance(request, dict):
-        return
-    request.pop(part, None)
+        if hasattr(request, part):
+            request.ClearField(part)
+        else:
+            return
+    else:
+        request.pop(part, None)
 
 
 def validate(tmpl, path):
@@ -237,7 +247,7 @@ def validate(tmpl, path):
     return True if re.match(pattern, path) is not None else False
 
 
-def transcode(http_options, **request_kwargs):
+def transcode(http_options, message=None, **request_kwargs):
     """Transcodes a grpc request pattern into a proper HTTP request following the rules outlined here,
     https://github.com/googleapis/googleapis/blob/master/google/api/http.proto#L44-L312
 
@@ -248,18 +258,20 @@ def transcode(http_options, **request_kwargs):
              'body'      (str): The body field name (optional)
              (This is a simplified representation of the proto option `google.api.http`)
 
+         message (Message) : A request object (optional)
          request_kwargs (dict) : A dict representing the request object
 
      Returns:
          dict: The transcoded request with these keys,
              'method'        (str)   : The http method
              'uri'           (str)   : The expanded uri
-             'body'          (dict)  : A dict representing the body (optional)
-             'query_params'  (dict)  : A dict mapping query parameter variables and values
+             'body'          (dict | Message)  : A dict or a Message representing the body (optional)
+             'query_params'  (dict | Message)  : A dict or Message mapping query parameter variables and values
 
      Raises:
          ValueError: If the request does not match the given template.
     """
+    transcoded_value = message or request_kwargs
     for http_option in http_options:
         request = {}
 
@@ -268,27 +280,35 @@ def transcode(http_options, **request_kwargs):
         path_fields = [
             match.group("name") for match in _VARIABLE_RE.finditer(uri_template)
         ]
-        path_args = {field: get_field(request_kwargs, field) for field in path_fields}
+        path_args = {field: get_field(transcoded_value, field) for field in path_fields}
         request["uri"] = expand(uri_template, **path_args)
-        # Remove fields used in uri path from request
-        leftovers = copy.deepcopy(request_kwargs)
-        for path_field in path_fields:
-            delete_field(leftovers, path_field)
 
         if not validate(uri_template, request["uri"]) or not all(path_args.values()):
             continue
 
+        # Remove fields used in uri path from request
+        leftovers = copy.deepcopy(transcoded_value)
+        for path_field in path_fields:
+            delete_field(leftovers, path_field)
+
         # Assign body and query params
         body = http_option.get("body")
 
         if body:
             if body == "*":
                 request["body"] = leftovers
-                request["query_params"] = {}
+                if message:
+                    request["query_params"] = message.__class__()
+                else:
+                    request["query_params"] = {}
             else:
                 try:
-                    request["body"] = leftovers.pop(body)
-                except KeyError:
+                    if message:
+                        request["body"] = getattr(leftovers, body)
+                        delete_field(leftovers, body)
+                    else:
+                        request["body"] = leftovers.pop(body)
+                except (KeyError, AttributeError):
                     continue
                 request["query_params"] = leftovers
         else: