Specify the instance grouping key in the message body instead of the URL

updated with a clearer suggestion of a solution

Feature request

Permit specifying the instance in the body of the message and that it can be used as a grouping key in addition to the job name.

NxLog(for example) which can send http output to pushgateway, cannot have dynamic variables in the URL itself. This would provide a more open and flexible method of sending data to the pushgateway.

The message body would contain a label which would be considered as a grouping key in addition to the job.
This label should have strict rules in how it can be recognized as an additionnal grouping key. (label prefix name, particular label name. or other method.

Bug Report

What did you do?

Configure NxLog to send data:

http://10.10.0.1:9091/metrics/job/job_name/
metricname{instance="1234",label2="value2"} 50

What did you expect to see?

Grouping key job_name, instance.

What did you see instead? Under which circumstances?
Grouping key is only: job_name
Instance in this case is considered a regular label and not a grouping key.

If specifying a label with a particular name or semantic (KEY_name, or instance) in the body, I would expect it to be used as an additionnal grouping key.

Ubuntu 18.04LTS, pushgateway version 1.2

The issue names the Pushgateway HTTP endpoint and metric message body, but no repository files or tests. Start by tracing how the endpoint parses grouping keys from the URL and metric labels, then clarify the recognition rule for a body label. Done means the behavior and its strict label semantics are agreed and covered by tests.