Proposal: version control system (VCS) information: revision

[jschaf]

Proposal: add vcs.revision as a semantic convention key.

It's often useful to add the version control revision, like a Git commit hash, to logs and traces that indicate the commit that the binary was built from. The vcs.revision is helpful to debug production issues.

As a counterpoint, service.version might already include the information or be detailed enough to identify the commit.

Complicating matters: should there also be vcs.repo to denote which repo the commit originated from?