client/service/src/builder.rs: Add build_info metric (#5192)

* client/service/src/builder.rs: Add build_info metric

Add static Prometheus metric exposing the chain name, the version and
the commit.

* client/service/src/builder.rs: Move node_role to static metrics

The Prometheus metrics `node_role` is static and thus there is no need
to keep a reference of it within `ServiceMetrics`. This follows the
example of the `build_info` metric.

* client/service/src/builder.rs: Adjust indentation