agent: introduce path allow list for requests going through the metrics proxy

[rboyer]

Added a new option ui_config.metrics_proxy.path_allowlist. This defaults to ["/api/v1/query", "/api/v1/query_range"] when the metrics provider is set to prometheus.

Requests that do not use one of the allow-listed paths (via exact match) get a 403 Forbidden response instead.

checklist

• Add the field to the Config struct (or an appropriate sub-struct) in
  agent/config/config.go.
• Add the field to the actual RuntimeConfig struct in
  agent/config/runtime.go.
• Add an appropriate parser/setter in agent/config/builder.go to
  translate.
• Add the new field with a random value to both the JSON and HCL blobs in
  TestFullConfig in agent/config/runtime_test.go, it should fail now, then
  add the same random value to the expected struct in that test so it passes
  again.
• Add the new field and it's default value to TestSanitize in the same
  file. (Running the test first gives you a nice diff which can save working
  out where etc.)
• If your new config field needed some validation as it's only valid in
  some cases or with some values (often true).
  • Add validation to Validate in agent/config/builder.go.
  • Add a test case to the table test TestConfigFlagsAndEdgeCases in
    agent/config/runtime_test.go.
• If your new config field needs a non-zero-value default.
  • Add that to DefaultSource in agent/config/defaults.go.
  • Add a test case to the table test TestConfigFlagsAndEdgeCases in
    agent/config/runtime_test.go.
• If your config should take effect on a reload/HUP.
  • Add necessary code to to trigger a safe (locked or atomic) update to
    any state the feature needs changing. This needs to be added to one or
    more of the following places:
    • ReloadConfig in agent/agent.go if it needs to affect the local
      client state or another client agent component.
    • ReloadConfig in agent/consul/client.go if it needs to affect
      state for client agent's RPC client.
  • Add a test to agent/agent_test.go similar to others with prefix
    TestAgent_reloadConfig*.
• Add documentation to website/pages/docs/agent/options.mdx.

[banks]

LGTM. Great job on the tests and validation 👍

[hashicorp-ci]

🍒✅ Cherry pick of commit a66c459 onto release/1.9.x succeeded!