Add support for metrics' insertion via Graphite protocol

[valyala]

The issue

Many users want switching from Graphite to Prometheus, since Prometheus has better query language, has better support for metric labels and has better storage for high metric volumes. But these users don't want switching from Graphite-compatible agents to Prometheus exporters.

The solution

Add support to VictoriaMetrics for metrics' insertion via Graphite protocols, so the Graphite data stored in VictoriaMetrics could be queried via PromQL.

[valyala]

Single-server VictoriaMetrics v1.6.0+ accepts Graphite plaintext protocol messages if -graphiteListenAddr flag is set.

It is unclear how to add support for Graphite protocol in the SaaS version, since the protocol has no authorization fields :(

[tenmozes]

implemented