This commits adds storage part and cluster RPC methods for metrics metadata.
Key concepts:
* vmstorage persists metadata in-memory only.
* vmstorage evicts metadata records older than 1 hour.
* vmstorage stores only the last value of metadata for time series metric name.
* vminsert opens an additional TCP connection to the vmstorage for metrics metadata write requests.
* vmselect implements prometheus compatible HTTP API for reading metrics metadata
This feature is available optional and must be enabled via flag - `-enableMetadata` provided to vminsert/vmsingle.
Fixes github.com/VictoriaMetrics/VictoriaMetrics/issues/2974
Metrics Metadata feature must also include RPC implementation. New RPC
server added stub for it, but it will work incorrectly if it'd be used
by different releases of vminsert/vmstorage.
This commit adds new RPC protocol for vminsert-vmstorage communication,
it acts in the same way as vmselect-vmstorage RPC.
It's implemented with new handshake hello methods in a backward
compatible way. Server attempts to parse RPC only if client send new
Hello message, while client fallbacks to the old Hello message if server
closes connection.
This change is need for the new metrics metadata forwarded from vminsert
into vmstorage.
Related issue:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2974
Changes extracted from PR:
https://github.com/VictoriaMetrics/VictoriaMetrics/pull/9487
