This "brings back" protobuf parsing, with the only goal to play with
the new sparse histograms.
The Prom-2.x style parser is highly adapted to the structure of the
Prometheus text format (and later OpenMetrics). Some jumping through
hoops is required to feed protobuf into it.
This is not meant to be a model for the final implementation. It
should just enable sparse histogram ingestion at a reasonable
efficiency.
Following known shortcomings and flaws:
- No tests yet.
- Summaries and legacy histograms, i.e. without sparse buckets, are
ignored.
- Staleness doesn't work (but this could be fixed in the appender, to
be discussed).
- No tricks have been tried that would be similar to the tricks the
text parsers do (like direct pointers into the HTTP response
body). That makes things weird here. Tricky optimizations only make
sense once the final format is specified, which will almost
certainly not be the old protobuf format. (Interestingly, I expect
this implementation to be in fact much more efficient than the
original protobuf ingestion in Prom-1.x.)
- This is using a proto3 version of metrics.proto (mostly to be
consistent with the other protobuf uses). However, proto3 sees no
difference between an unset field. We depend on that to distinguish
between an unset timestamp and the timestamp 0 (1970-01-01, 00:00:00
UTC). In this experimental code, we just assume that timestamp is
never specified and therefore a timestamp of 0 always is interpreted
as "not set".
Signed-off-by: beorn7 <beorn@grafana.com>
