Ephemeral storage

[pstibrany]

We would like to introduce support for "ephemeral storage" in Mimir. This is in-memory storage of series in ingesters. Series are removed from ephemeral storage after a short period of time, and are queryable via special selector in PromQL queries.

Mimir Distributor will mark some series as "ephemeral" before forwarding them to ingester, and ingester will instantiate ephemeral storage on per-tenant basis when needed.

This issue will track PRs related to introduction of this feature into Mimir:

• Update of Protobuf message between distributor and ingester
  • Add ephemeral_timeseries to WriteRequest #3889
• Introduction of ephemeral storage to Ingester
  • Extract pushStats from PushWithCleanup #3913
  • Extract pushSamplesToAppender method from PushWithCleanup. #3915
  • Add ephemeral storage to ingester #3922
  • Make ephemeral storage queryable #3961
  • Limits and errors for ephemeral storage #4004
  • Fix truncation of ephemeral storage if there are no persistent series #4052
• marking of series as ephemeral in Distributor
  • Mark series ephemeral in distributor based on series matchers in runtime config #3897
  • introduce ephemeral checker service and make distributor service depend on it #3997
• CHANGELOG for ephemeral storage #4024
• Do not call Truncate() on the ephemeral TSDB head every HeadCompactionInterval (1m) but every retention period to reduce the pressure
• documentation
• dashboards
• alerts

[pstibrany]

We're not going to develop this feature further. To simplify the codebase, we're removing it in #4252.