[ENG-10665] support osfmetrics migration

[aaxelb]

• improve test setup/teardown, isolating test runs with index-name prefixes
• index CyclicRecord by cycle_coverage, not by time created
• more consistent date formatting in index names (use semver-like 2026.3.31 to align with the timeseries_timeparts field)
• move djelme_registry.all_recordtypes to semi-private attr (require interacting with registry thru methods, not direct access to mutable dict)
• improve docs

[felliott]

looks good, no notes!