Logging: add support for additional 'LogEntry' fields (#6229)

Use namedtuples to reduce boilerplate in 'logger.Batch',  'logger.Logger', and 'entries' implementations:  remove 'entry_type' and 'PAYLOAD_KEY' warts, dispatching instead based on class to load / save payload.

Parse 'LogEntry.receiveTimestamp' -> 'received_timestamp' attr.  Make 'received_timestamp' default to 'None' for all instances. It is set only when parsing a server response.

Add support for 'LogEntry.traceSampled' field.

Add support for 'source_location' field of log entries.

Add support for 'operation' field of log entries.

Add 'ProtobufEntry.payload_json': Returns 'None' if the payload is a protobuf message.    'ProtobufEntry.payload_pb' now returns 'None' if the payload is *not* a protobuf message.

Drop 'EmptyEntry':  just use 'LogEntry'.

Closes #5601.
Closes #6094.

Author: tseaver

docs/entries.rst

@@ -4,3 +4,4 @@ Entries
 .. automodule:: google.cloud.logging.entries
   :members:
   :show-inheritance:
+  :member-order: groupwise

google/cloud/logging/_helpers.py

@@ -16,7 +16,7 @@
 
 import requests
 
-from google.cloud.logging.entries import EmptyEntry
+from google.cloud.logging.entries import LogEntry
 from google.cloud.logging.entries import ProtobufEntry
 from google.cloud.logging.entries import StructEntry
 from google.cloud.logging.entries import TextEntry
@@ -54,7 +54,7 @@ def entry_from_resource(resource, client, loggers):
     if 'protoPayload' in resource:
         return ProtobufEntry.from_api_repr(resource, client, loggers)
 
-    return EmptyEntry.from_api_repr(resource, client, loggers)
+    return LogEntry.from_api_repr(resource, client, loggers)
 
 
 def retrieve_metadata_server(metadata_key):