Bridge: fixed relayer version metric value (#4492)

Before relayer crates have been moved + merged, the `MetricsParams` type
has been created from a `substrate-relay` crate (binary) and hence it
has been setting the `substrate_relay_build_info` metic value properly -
to the binary version. Now it is created from the
`substrate-relay-helper` crate, which has the fixed (it isn't published)
version `0.1.0`, so our relay provides incorrect metric value. This
'breaks' our monitoring tools - we see that all relayers have that
incorrect version, which is not cool.

The idea is to have a global static variable (shame on me) that is
initialized by the binary during initialization like we do with the
logger initialization already. Was considering some alternative options:
- adding a separate argument to every relayer subcommand and propagating
it to the `MetricsParams::new()` causes a lot of changes and introduces
even more noise to the binary code, which is supposed to be as small as
possible in the new design. But I could do that if team thinks it is
better;
- adding a `structopt(skip) pub relayer_version: RelayerVersion`
argument to all subcommand params won't work, because it will be
initialized by default and `RelayerVersion` needs to reside in some util
crate (not the binary), so it'll have the wrong value again.