[ENG-9697] index granularity

[aaxelb]

• configurable timeseries index granularity (for elastic8 and forward; elastic6 left as-is)
  • instead of arbitrary date patterns, use more constrained "timedepth", a count of the time values used in timeseries index names (e.g. 1 for yearly "YYYY", 2 for monthly "YYYY_MM", etc)
  • per-app setting: DJELME_DEFAULT_TIMEDEPTH (default 3, daily indexes)
  • per-recordtype setting:

    class MyRecord(elasticsearch_metrics.imps.elastic8.EventRecord):
        class Meta:
            timedepth = 2  # monthly indexes for this type

• improve and tidy docs, tests, comments, annotations

[felliott]

Glorious!