Add OpenMetrics support

[n-peugnet]

Hi, I wanted to use prom2json to convert the output of promtool tsdb dump-openmetrics, but I get the following error:

error reading metrics: reading text format failed: text format parsing error in line 1: expected integer as timestamp, got "1739184970.553"

Here is the corresponding line:

go_gc_duration_seconds{instance="node5",job="file_discovery",quantile="0.0"} 0 1739184970.553

As I understand it, this comes from a breaking change between Prometheus exposition format and the OpenMetrics one that the timestamp must be in seconds.

Is parsing OpenMetrics format in the scope of prom2json?

[n-peugnet]

OK, so from what I understand, this is currently blocked by prometheus/common#669.

[beorn7]

Yes, sounds about right.