Adds the `metricsdiff` tool. The metricsdiff tool parses two files containing prometheus metrics and calculates the sets of metrics that were added or removed between the two files or have changed labels. This tool is added to ensure that the metrics been generated for `metricsgen` match the bespoke metrics.
The following metrics were found to be different between master and the the tool was built with. The output makes sense given that the metrics branch does _not_ contain https://github.com/tendermint/tendermint/pull/8480.
```
./metricsdiff metrics_master metrics_generated
Removes:
--- tendermint_consensus_proposal_create_count
--- tendermint_consensus_vote_extension_receive_count
--- tendermint_consensus_round_voting_power_percent
--- tendermint_consensus_proposal_receive_count
```
This pull requests adds a new tool, metricsgen, for generating Tendermint metrics constructors from `Metrics` struct definitions. This tool aims to reduce the amount of boilerplate required to add additional metrics to Tendermint.
Its working is fairly simple, it parses the go ast, extracts field information, and uses this field information to execute a go template.
This pull request also adds a proof-of-concept of the tool's output and working by using it to generate the [indexer metrics](https://github.com/tendermint/tendermint/pull/8479/files#diff-4b0c597b6fa05332a2f9a8e0ce079e360602942fae99dc5485f1edfe71c0a29e) using `//go:generate` directives and a simple `make` target.
The next steps for this tool are documented in https://github.com/tendermint/tendermint/issues/8485 and https://github.com/tendermint/tendermint/issues/8486, which detail using the tool to generate the `metrics.md` documentation file and using the tool to migrate away from `go-kit`.
William BanfieldandGitHub