Introduce near_build_info with a constant value 1 which exports
node’s build information as labels.  Specifically, release, build and
rust version are exported.  See [1] for description how this metric
can be used.

[1] https://www.robustperception.io/exposing-the-software-version-to-prometheus

While at it, set near_node_protocol_version and near_node_db_version
only once (since they are always set to the same value) rather than
doing so each time statistics are printed.  Added bonus of this is
that those metrics are now set immediately when node starts rather
than the first time the stats info line is printed in the log.

Issue: near#5979