Commit 71bb9fc0d0 introduced a regression.
If labels are empty and relabeling is not configured, influx ingestion hanlder
performed an earlier exit due to TryPrepareLabels call.
Due micro-optimisations for this procotol, this check was not valid.
Since it didn't take in account metircName, which added later and skip metrics line.
This commit removes `TryPrepareLabel` function call from this path and inline it instead.
It properly track empty labels path.
Adds initial tests implementation for data ingestion protocols.
Related issue:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7933
Signed-off-by: f41gh7 <nik@victoriametrics.com>
b745f8eb2b