Add support for sourced metrics. (#6895)

* Add support for sourced metrics.

A sourced metric is a metric that obtains its values
from an existing source, rather than the values being
independently recorded. It thus allows collecting
metrics from existing counters or gauges without
having to duplicate them in a dedicated prometheus
counter or gauge (and hence another atomic value).

The first use-case is to feed the bandwidth counters
from libp2p directly into prometheus.

* Tabs, not spaces.

* Tweak bandwidth counter registration.

* Add debug assertion for variable labels and values.

* Document monotonicity requirement for sourced counters.

* CI

* Update client/network/src/service.rs

Co-authored-by: Max Inden <[email protected]>

Co-authored-by: Max Inden <[email protected]>