Add local telemetry testing guide

[shaaza]

Summary

• expand custom metrics docs with a section on testing locally

Testing

• npm i -g mintlify (fails: 403 Forbidden)
• mintlify broken-links (fails: command not found)

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https://chatgpt.com/codex/tasks/task_b_6842286154d4832d8af5b18a1bd50a67