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: https://github.com/googleapis/python-bigtable/issues/1004