Add prometheus to client and begin implementing custom metrics

[am1r021]

This PR begins the Prometheus integration work by first adding a Prometheus client server to the ethereumjs client and then adding custom metrics discussed in #3089.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.93%. Comparing base (4be68d2) to head (961afba). Report is 9 commits behind head on master.

:exclamation: Current head 961afba differs from pull request most recent head 39d1daa. Consider uploading reports for the commit 39d1daa to get more accurate results

Additional details and impacted files

Flag	Coverage Δ
block	88.34% <ø> (+6.10%)	:arrow_up:
blockchain	91.61% <ø> (+0.68%)	:arrow_up:
client	?
common	98.25% <ø> (+4.23%)	:arrow_up:
devp2p	82.12% <ø> (+0.75%)	:arrow_up:
ethash	∅ <ø> (?)
evm	74.33% <ø> (+1.03%)	:arrow_up:
genesis	99.98% <ø> (ø)
statemanager	77.02% <ø> (+2.30%)	:arrow_up:
trie	89.29% <ø> (+29.69%)	:arrow_up:
tx	95.45% <ø> (+9.74%)	:arrow_up:
util	89.13% <ø> (+7.81%)	:arrow_up:
vm	80.20% <ø> (+18.64%)	:arrow_up:
wallet	88.35% <ø> (+1.10%)	:arrow_up:

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[acolytec3]

Changes I have made to the original PR

• Add a new CLI flag for prometheusPort
• Moved prometheusMetrics to a new optional property on the config object that is passed around within the client. This should make for less repetitive code when we add new metrics in other places within the client
• Add better typing to stopClient parameters in bin/cli.ts
• Add minimal docs for how to start metrics server