support customizing routing metadata

[daniel-sanche]

The gapic generator automatically attaches routing metadata based on the protos, by appending the x-goog-request-params to whatever metadata the user passes in. But sometimes users may want to customize the x-goog-request-params field itself. In bigtable, we need to customize the field based on some values set in the veneer layer

To solve this, I propose we add a check to ensure we are only setting x-goog-request-params when it doesn't exist in the passed in metadata, to avoid adding a duplicate field. But let me know if you have other thoughts

Related bigtable bug: googleapis/python-bigtable#1004

[daniel-sanche]

Quick update on this

In #2133 (comment), parthea suggested we should look at changing the AIP before making this change.

But with the changes in #2133, this may no longer be needed. Now that async routing is working properly at the gapic level, we can likely solve the duplicate metadata issue by just removing our hand-crafted routing metadata. So I'm going to look at that next

I still think it's a reasonable behavior to avoid adding routing fields if the caller passes their own, but if I can avoid our immediate issue in another way, that's fine with me

[daniel-sanche]

I think I'm going to be able to close this, and fix the issue on the bigtable level