Add metadata_overlay

[evaogbe]

To be used in the routing header like this:

self._create_book = api_callable.create_api_call(
self.library_service_stub.CreateBook,
settings=defaults['create_book'],
metadata_overlay=lambda req: {
b'x-google-request-params':
b'name={}&book.read={}'.format(req.name, req.book.read)})

[theacodes]

Hey @eoogbe, should this be done against api_core instead? (we're about to drop gax)

[evaogbe]

@jonparrott When you say "about to", how soon do you mean? Routing headers are pretty critical path.

[theacodes]

@lukesneeringer and @landrito can speak to that.

[theacodes]

@eoogbe I also thought about this a while and I can't see why we need this. Metadata can already be specified by gapic clients in the generated method.

For example, we could update this generated code:

defcreate_book(self, name, read=False, ...):
request=book_pb2.Book(
name=name,
read=read,
)
returnself._create_book(request, ...)

to just this:

defcreate_book(self, name, read=False, ...):
request=book_pb2.Book(
name=name,
read=read,
)
metadata= [
('x-google-request-params',
'name={}&book.read={}'.format(request.name, request.read)),
]
returnself._create_book(
request, metadata=metadata, ...)

Therefore obviating the need for this complex "overlay" logic. Am I missing something?

[evaogbe]

@jonparrott Fair enough

[theacodes]

Cool - glad we could figure out a simpler solution. :)