Some metrics protocol changes (#806)

* Some metrics protocol changes

- Add an envelope time stamp. That should be timestamp when the packet
  is actually sent.
- Remove `start_timestamp`/`end_timestamp` from `TimeseriesMetric` as it
  is not clear how it will be used. Can add later if needed.

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* change comment

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* clean up

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* comment to mention absolute time

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* adding server normalized timestamps

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* Changeset

* remove incorrect changeset

* try again

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Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

.changeset/sour-carrots-burn.md

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+---
+"github.com/livekit/protocol": patch
+"@livekit/protocol": patch
+---
+
+Metrics server side timestamp