Add a metrics gauge for commits.

Will be useful for setting alerts when ingestions/commits or somesuch falls to zero, i.e.we've stopped ingesting.

BUG=skia:

Review URL: https://codereview.chromium.org/671973002
perfserver: Add a constant for the instance name.

That way, if we happen to rename it to something else, we don't need to
change in four different scripts.

Just change vm_config.h and we are done.

BUG=None
TEST=None
R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/665913006
perf: Provide Avg() function.

It should be just an alias for Ave() function for average in
calculations.

To test this:

$ cd ~/golib/src/
$ go get -u -t skia.googlesource.com/buildbot.git/perf/go/...
$ cd skia.googlesource.com/buildbot.git/perf/
$ make testgo

BUG=None
TEST=see above
R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/656483004