feat(x/metrics): add module for emiting custom chain metrics (backport #1668)

[mergify]

This is an automatic backport of pull request #1668 done by Mergify.
Cherry-pick of 9a0aed7 has failed:

On branch mergify/bp/release/v0.18.x/pr-1668
Your branch is up to date with 'origin/release/v0.18.x'.

You are currently cherry-picking commit 9a0aed76.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   cmd/kava/cmd/app.go
	new file:   x/metrics/abci.go
	new file:   x/metrics/abci_test.go
	new file:   x/metrics/module.go
	new file:   x/metrics/spec/README.md
	new file:   x/metrics/types/keys.go
	new file:   x/metrics/types/metrics.go
	new file:   x/metrics/types/metrics_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   app/app.go
	both modified:   go.mod

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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