Duplicate x-goog-request-params routing header

[daniel-sanche]

The library uses _make_metadata to populate the x-goog-request-params metadata field. Later, the gapic layer automatically attaches its version of the same field. But the gapic-side doesn't touch app_profile_id

Both headers are sent to the backend, which can cause issues with routing

We can resolve this by making a patch in the gapic_generator

[daniel-sanche]

Re-opening this. After trying to fix this at the gapic-generator level, it looks like we need to make some fixes back at the bigtable level. Context here: googleapis/gapic-generator-python#2078 (comment)