Trace: add client_info support to clients.

[tseaver]

Toward #7825.

[tswast]

Linter failing (needs to be blackened).

Coverage failing

coverage report --show-missing --fail-under=100
Name Stmts Miss Branch BrPart Cover Missing
-------------------------------------------------------------------------------
google/cloud/trace/__init__.py 12 0 0 0 100%
google/cloud/trace/_gapic.py 55 0 16 0 100%
google/cloud/trace/client.py 19 0 2 0 100%
google/cloud/trace/v1/__init__.py 3 0 0 0 100%
google/cloud/trace/v1/_gapic.py 38 2 0 0 95% 164-165
google/cloud/trace/v1/client.py 32 0 12 0 100%
google/cloud/trace_v1/__init__.py 8 0 0 0 100%
google/cloud/trace_v1/types.py 19 0 8 0 100%
google/cloud/trace_v2/__init__.py 8 0 0 0 100%
google/cloud/trace_v2/types.py 23 0 8 0 100%
-------------------------------------------------------------------------------
TOTAL 217 2 46 0 99%

https://source.cloud.google.com/results/invocations/97c40a9f-2a86-4639-b98f-11a5b146a23f/targets/cloud-devrel%2Fclient-libraries%2Fgoogle-cloud-python%2Fpresubmit%2Ftrace/log