Add metrics

[unnawut]

Consolidating metrics requirements from various sources. Some of them are currently available but only because we're parsing it from logs and needs conversion to proper metrics.

• [ ] Count of calls to ethereum client (we already have per calling signature but not the total, which makes it hard to sum up)
• [ ] Count of deposits created (total and per token address)
• [x] Count of blocks submitted https://github.com/omgnetwork/elixir-omg/pull/1649
• [ ] Count of txs submitted per token address excluding fee txs
• [ ] Count of txs submitted excluding fee transactions (txs submitted already exists but it includes fee txs)
• [x] Count of utxos in utxoset (sampled at regular time intervals) - https://github.com/omgnetwork/elixir-omg/pull/1666
• [x] Count of unique addresses owning at least 1 utxos (sampled at regular time intervals) - https://github.com/omgnetwork/elixir-omg/pull/1666
• [ ] Count of exit events
  • [ ] start exit
  • [ ] piggyback IFE
  • [ ] challenge exit
  • [ ] invalid exits
  • [ ] byzantine conditions
  • [ ] exit finalized
• [ ] Exit queues
  • [ ] Count of exits still under exit period, per exit queue
  • [ ] Count of exits ready to be processed, per exit queue
• [ ] Attempted gas price (currently available only in logs)
• [ ] Successful gas price (currently available only in logs)
• [ ] Childchain block number
  • [x] Last submitted by child chain https://github.com/omgnetwork/elixir-omg/pull/1649
  • [ ] Last recognized by watcher/watcher_info
• [x] Balance per token (currently count by utxos and therefore missing deposits) - https://github.com/omgnetwork/elixir-omg/pull/1666