client/network/src/protocol: Refactor Prometheus metric logic (#5266)

* client/network/src/protocol: Start Prometheus metric help with capital

* client/network/src/protocol: Differentiate metric status as label

Prometheus query language is powerful through its multi-dimensional data
model. Metric names are hirarchical whereas labels enable data to become
multi-dimensional.

Exposing the justification of finality-proof status as a label allows
for more powerful queries.

* client/network/src/protocol: Remove 'Total' from non counter metric help

The word 'total' is reserved for accumulating counters. Counters have to
be monotonically increasing. `obsolete_requests` can decrease, thereby
it is defined as a `Gauge` and not a `Counter`.

For more details on metric naming see
https://prometheus.io/docs/practices/naming/