feat: add an inline OM created timestamp syntax

[Maniktherana]

this PR aims to update the created timestamp syntax as outlined in the following proposal: prometheus/proposals#43

[ArthurSens]

Nice, thanks for starting this!

I think we may want a different approach though, OpenMetrics 1.0 is not going away, so we need to keep the _created lines for users of that version of the protocol.

What we want is a separate implementation for 2.0.

[Maniktherana]

Nice, thanks for starting this!

I think we may want a different approach though, OpenMetrics 1.0 is not going away, so we need to keep the _created lines for users of that version of the protocol.

What we want is a separate implementation for 2.0.

understood, how can we manage both implementations together? Do we use a feature flag for this?

[ArthurSens]

Feedback from our call:

Clients who use prometheus/common to encode data use NewEncoder and it returns a different encoder based on Content Type.

We can try two options and see what works best:

• Create a totally new encoder MetricFamilyToOpenMetrics2.0
  or
• The existing MetricFamilyToOpenMetrics encoder could have a new encoder option that decides its version