Include/Maintain version info in exporters

For all of us that fix issues in our setup before Percona includes them in the official version it would be nice, if you kept version information in the exporters.

For example:
{{
./mysqld_exporter --version
mysqld_exporter, version (branch: , revision: )
build user:
build date:
go version: go1.15.7
}}

As you can see, there is no clue about which version is this one, which tag/commit/branch in the repo is this exporter based on.