Extend OpenMetrics by a stability data type

In order to improve metrics experience, I would like to propose two fields to be added to the spec.

Stability

Stability is a string that is  used to describe the maturity of the MetricFamily. If it is unset, the MetricFamily is considered stable
Recommended values for lifecycle:
* Alpha for metrics that might be renamed or are unstable in terms of other features (e.g. units or dimensions).
* Beta for metrics that will be considered stable soon.
* Stable This is the default and what consumers can assume if the Stability string is not set.
* Deprecated for metrics that should not be used anymore. 

StabilityHint

StabilityHint is string that is used to provide a human-readable hint for the MetricFamily. This can include e.g. a replacement for the MetricFamily.

As an example:

# TYPE process_cpu_microseconds_total counter
# UNIT process_cpu_microseconds_total seconds
# HELP process_cpu_microseconds_total Total user and system CPU time spent in seconds.
# STABILITY process_cpu_microseconds_total Deprecated
# STABILITYHINT process_cpu_microseconds_total This metric going to be replaced by process_cpu_seconds_total.
process_cpu_microseconds_total 4.20072246e+06

This could allow consumers to warn when an unstable metric is used and improve metrics lifecycle. We currently see issues where e.g. grafana dashboards use a deprecated metric and users only figure out when the metric is gone on the consumer side.

This is similar to https://github.com/OpenObservability/OpenMetrics/issues/189 but with a clearer focus on lifecycle.

Start by reading the OpenMetrics specification and the related OpenObservability issue #189, since no implementation files or tests are named here. The work is done when the lifecycle fields, syntax, semantics, and compatibility implications have been agreed and incorporated into the specification.