OM 2.0: Created Timestamps text exposition format

[ArthurSens]

When implementing Created Timestamp support in Prometheus OM's parser, we've faced many challenges with the current format:

1. If the parser was not ready to parse created timestamps, how they are exposed looks precisely the same as how a usual metric looks. CT would be ingested as a metric and could lead to a 50% increase in metric ingestion if the target exposes mainly counters.
2. Since they look like metrics, parsers need extra logic to differentiate actual metrics that end with the _created suffix.
3. No strict ordering requires state handling for complex metric types like histograms and summaries.

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We'd love to address all these problems with OpenMetrics 2.0. In the last Prometheus CT WG call, we discussed OM for a bit and came up with the following ideas:

1. Expose CT similarly to the way we expose metadata (with a comment):

# TYPE example_metric counter
# HELP example_metric An example counter with a created timestamp
# CT example_metric 1234
example_metric 1
# CT example_metric{foo="bar"} 4312
example_metric{foo="bar"} 2

2. Expose CT inline with the metric

# TYPE example_metric counter
# HELP example_metric An example counter with a created timestamp
example_metric 1 ct1234
example_metric{foo="bar"} 2 ct4312

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We're opening this issue to discuss both ideas or new ones if anyone is thinking of another approach.

[bwplotka]

Proposal @Maniktherana created prometheus/proposals#43

[krajorama]

PR prometheus/docs#2636