prometheus metric unification: remove chain name metric prefix

[gabreal]

Prometheus metrics are currently being prefixed with polkadot_, substrate_ or differently depending on the chain name. This makes dashboards based on these metrics unnecessarily restricted to a certain chain even though the metrics represent the same kind of information e.g. polkadot_block_height vs substrate_block_height. creating graphs and alerts for both kinds of chains would require writing queries like {__name__=~"(polkadot|substrate)_block_height"}. This can be avoided using a chain label indicating the network the client is operating in and for compatibiltiy reasons allow only the polkadot_ prefix to remain.

Changing this will make dashboards and existing alerts more universally usable and removes the ambiguity where these metrics belong to resp originate from.

Such a chain label could be directly added to the constructor: https://docs.rs/prometheus/0.12.0/prometheus/struct.Registry.html#method.new_custom

see also https://github.com/paritytech/devops/issues/1013

[gabreal]

resolved by paritytech/polkadot#3623 and #9543