Is your feature request related to a problem? Please describe.
This feature request is to support the Prometheus Metric Metadata API at /api/v1/metadata in Prometheus v2.15.0+. Similar to #643, but this endpoint returns metadata about scraped metrics without any associated target information.
vmselect currently returns an empty placeholder response for this endpoint (see e6bf88a).
More generally, the request is for VictoriaMetrics to support metric metadata throughout its Prometheus-compatible components. In addition to implementing /api/v1/metadata, metadata would need to be scraped from Prometheus targets and also propagated via Prometheus remote-write, using the metadata field added to the Prometheus remote-write protocol in v2.23.0.
Describe the solution you'd like
A full solution for Prometheus metadata would require updates to a few components:
vmselect implements the /api/v1/metadata endpoint with a response containing at least one metadata object per metric name.vmstorage adds support for storing metric metadata.
_prometheus_metadata) with metadata stored in metric, type, help, unit labels, requiring no change to vmstorage itself.vmagent scrapes metadata from Prometheus targets, and add the metadata field to remote_write requests it sends.vmagent and vminsert extract the metadata field from remote_write requests they receive.Describe alternatives you've considered
Since Victoriametrics currently ignores Prometheus metric metadata, the only alternative would be to proxy this endpoint to another handler which serves metric metadata stored in another system.
Additional context
Since the introduction of the Metric Metadata API, metric metadata has become an increasingly critical part of the Prometheus user experience.
One key use-case related to this feature request is to support Grafana's in-line metrics help text when using VictoriaMetrics as a Prometheus datasource:
dddeeemmm, 7840vz, kdask, wbh1, dfredell and 12 morearctan90 and ns-rorsten
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