Support extended percentiles for application metrics

[sentry-junior]

Application Metrics distribution queries currently expose only P50, P75, P90, P95, and P99 in dashboards and the query backend. This prevents dashboards from representing tail values such as P99.9 and P99.99, and editing a widget payload directly does not provide a workaround.

• The supported aggregations are documented in Application Metrics.
• The backend defines fixed percentile functions through P99 and does not define a custom percentile() function for Application Metrics (source).
• max is available but is not equivalent to an extended percentile.

via Michael Chai.

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View Junior Session [Sentry]

[linear-code]

ENG-8613