git: Add a gitversion label to command related metrics

Merged Siddharth Asthana requested to merge edith007/gitaly:issue_4049_edith into master

This MR aims to solve the issue #4094 (closed)

We sometimes want to know the git version of the spawned command.
Add gitversion label to the command metrics to increase the visibility of git version.

Edited by Siddharth Asthana