Enhancement: add prometheus exporter plugin that exposes key nodeos metrics

[matthewdarwin]

Copy from: EOSIO/eos#9902

or monitoring, nodeos should have a prometheus exporter plugin (that runs on a separate port), which exposes key metrics that would be useful for monitoring.

Metrics such as

what is returned from /v1/chain/get_info (head block number, lib)
unapplied transaction queue sizes
blacklisted transactions size
subjective billing sizes
scheduled transaction size
number of forks by producer
number of unapplied blocks by producer
number of dropped blocks by producer
number of missed blocks (missed in a round) by producer
number of missing producers (missed 12 blocks in a round) by producer
number of double production (more than 12 blocks in a round) by producer
average (and last) block arrival time by producer
number of transactions per block by producer
amount of blockchain cpu used per block by producer
number of bytes per block by producer
number of bad actors which have exceeded their transaction limit (as per the changes introduced in nodeos 2.0.9)
number of clients connected (inbound, outbound, failed)
number of api (failed) requests/sec (by request type)
uptime
cpu usage by thread
disk space used, by volume (blocks, ship, state, trace)
disk space available
rocksdb related
blockvault related (produced block, vs did not produce because other nodeos "won")
replay status (when starting from snapshot and replaying blocks)
etc..

Where possible, attribute actions to a specific producer (eg when counting dropped blocks, specify which producer missed those blocks)

[spoonincode]

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