prevent 0xa api_key suffix in api_key

[ohmayr]

Determine this is the right repository

• I determined this is the correct repository in which to report this bug.

Summary of the issue

if a 0xa character appears at the end of
the api_key, the underlying grpc implementation
will fail with an invalid value header error.

see: https://gitlab.uni-hannover.de/tci-gateway-module/grpc/-/blob/v1.9.x/src/core/lib/surface/validate_metadata.cc#L84

to reproduce this, simply add a 0xa character to an api_key and try to perform any generative action via grpc transport.

Related PR for more context: googleapis/google-cloud-python#13963.