VAULT-28329: Fix months activity log counts when querying for a namespace

[miagilepner]

Description

Previously, when you queried the activity log from a namespace you would get the following responses:

{
  "by_namespace": [{...}], // filtered by the request namespace
  "months": [{
    "counts": {...}, // NOT filtered
    "namespaces": [{...}], // filtered by the request namespace
    "new_clients": {
      "counts: {...}, // NOT filtered
      "namespaces": [{...}], // filtered by the request namespace
    }
  }],
  "total": {...} // filtered by the request namespace
}

This was very confusing, and also makes the UI a bad experience if you try to look at only one namespace.

This PR fixes this for the /activity and /activity/monthly endpoints.

Ent PR: https://github.com/hashicorp/vault-enterprise/pull/6260

TODO only if you're a HashiCorp employee

• Labels: If this PR is the CE portion of an ENT change, and that ENT change is
  getting backported to N-2, use the new style backport/ent/x.x.x+ent labels
  instead of the old style backport/x.x.x labels.
• Labels: If this PR is a CE only change, it can only be backported to N, so use
  the normal backport/x.x.x label (there should be only 1).
• 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! ✅

[VioletHynes]

While I agree that this fixes buggy behaviour, this is, strictly speaking, an API behaviour change -- so maybe this should be a change instead of an improvement?

[ccapurso]

I agree with this. The current output is inconsistent but it is always possible that someone has become dependent on the existing behavior.

[VioletHynes]

Approved! I do think this should be a change but I don't feel strongly enough to block the PR. Awesome fix!

[ccapurso]

I agree with this. The current output is inconsistent but it is always possible that someone has become dependent on the existing behavior.

[divyaac]

Looks good! You can probably push this because my only comments were involving the tests in the ENT pr!