Features/add initial subnet metrics

[cryptoAtwill]

Proposed Changes

Add initial metrics to track:

• [x] Number of created subnets
• [x] Number of killed subnets
• [x] Number of active subnets
• [x] Grafana dashboard
• [x] Documentation and scripts to setup

Sample dashboard:

Checklist

Before you mark the PR ready for review, please make sure that:

• [ ] All commits have a clear commit message.
• [ ] The PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, INTERFACE BREAKING CHANGE, CONSENSUS BREAKING, build, chore, ci, docs,perf, refactor, revert, style, test
  • area: api, chain, state, vm, data transfer, market, mempool, message, block production, multisig, networking, paychan, proving, sealing, wallet, deps
• [ ] This PR has tests for new functionality or change in behaviour
• [ ] If new user-facing features are introduced, clear usage guidelines and / or documentation updates should be included in https://lotus.filecoin.io or Discussion Tutorials.
• [ ] CI is green

[adlrocha]

We are making good progress. Thanks! I would try the event listener approach as part of this PR now that you know how to collect metrics. I think it can give all the skeleton that we need to collect "real-time" metrics.