Add Protobuf info in the agent header "x-goog-api-client"

[PierrickVoulet]

A basic value currently looks like "gl-php/7.2.16-1+ubuntu14.04.1+deb.sury.org+1 gapic/ gax/1.2.0 grpc/1.26.0". Two additional pieces of information would be great to add:

• Version of Protobuf
• Protobuf implementation (C or PHP)

[PierrickVoulet]

I did a quick analysis and found that the implementation comes from Gax so I opened a feature request.

It is technically possible to override the agent header value at client library-level to add the information but it would prefer to avoid.

[fiboknacky]

The feature request is already complete, so I close this issue.