Add metrics.proto to other protobuf code (prometheus#8996)

We cannot just use prometheus/client_model directly because we want to
stay consistent with the use of gogo-protobuf. So this converts
metrics.proto to proto3 and edits it lightly so that it fits into
the framework how prometheus/prometheus handles protobuf.

Note that metrics.proto couldn't be merged into the prompb package
because prompb already has an Exemplar type, which is different from
the Exemplar type in metrics.proto. The directory structure seems to
play a role in the protobuf world, so I better kept it.

Signed-off-by: beorn7 <beorn@grafana.com>