[Docs] Add usage metrics page

[yhyakuna]

Description

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🎫 Jira ticket

Although initial question was about what's new in Vault 1.18, the root issue was lack of information.

This PR adds "Usage metrics" page under Client count doc --> Content type is "How-to" guide

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[lucymhdavies]

Looks good. Can we get a little bit at the end with an example of what those CSV/JSONL files look like?

I can provide some examples from my own Vault if it helps.

[yhyakuna]

Looks good. Can we get a little bit at the end with an example of what those CSV/JSONL files look like?

I can provide some examples from my own Vault if it helps.

@lucymhdavies Yes, if you have it handy, could you please send it to me. I'll add it to the doc. 🙏

On second thought, the API doc has sample response. That may be enough? So, I added the link to API docs.

[lucymhdavies]

aha! Yes, the API response example from https://developer.hashicorp.com/vault/api-docs/system/internal-counters#activity-export is perfect.