Add client feature logging and metrics

Related https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14332

Example logs:

time="2017-10-05T13:40:10+01:00" 
level=info 
msg="spawn complete" 
args="[/usr/local/bin/git --git-dir /Users/andrewn/code/gitlab/gitlab-development-kit/repositories/moocow/html5-boilerplate.git log -z --pretty=format:%H%x1f%s%x1f%B%x1f%an%x1f%ae%x1f%aI%x1f%cn%x1f%ce%x1f%cI%x1f%P --max-count=1 a0073b9de64963a6ff91992d4b518eb0d87dba93 -- modernizr-config.json]" 
command.exitCode=0 
command.inblock=0 
command.maxrss=1970176 
command.oublock=0 
command.real_time_ms=10.458 
command.system_time_ms=1.9780000000000002 
command.user_time_ms=2.419 
grpc.meta.client_feature=bob 
grpc.meta.client_name=gitlab-ruby 
grpc.method=LastCommitForPath 
grpc.request.repoPath=moocow/html5-boilerplate.git 
grpc.request.repoStorage=default 
grpc.request.topLevelGroup=moocow 
grpc.service=gitaly.CommitService 
path=/usr/local/bin/git 
peer.address= 
span.kind=server 
system=grpc

(notice the new grpc.meta.client_feature and grpc.meta.client_name fields)

New metrics are also available in prometheus:

gitaly_feature_requests{client_feature="bob",client_name="gitlab-ruby",grpc_code="OK"} 146

This will give us insight into n+1 problems, feature-by-feature failures and more