A few things to prepare for deploying the Buzz push gateway as
implemented in #6269:
- Cut `buzz-push-gateway` chart `0.2.0` from the current config
(certificate auth, dogfood client)
- Datadog-compatible metrics
- Align the desired schema, metrics docs, and production ingress
guidance with the current dogfood deployment contract

## Links

- [BUZZ-17: Publish the MVP-compatible gateway
chart](https://linear.app/squareup/issue/BUZZ-17/publish-the-mvp-compatible-gateway-chart)
- [BUZZ-26: Add Datadog-compatible push-gateway metrics and correct
source
drift](https://linear.app/squareup/issue/BUZZ-26/add-datadog-compatible-push-gateway-metrics-and-correct-source-drift)

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Signed-off-by: Tom Brow <tomb@block.xyz>
Co-authored-by: Codex <noreply@openai.com>