Store metrics docstrings and allow to query them

[AlekSi]

Prometheus doesn't currently store metrics docstrings (#HELP lines in text format). They could be useful for a user in various UIs and dashboards.

I think they can be stored as simple non-versioned metainformation. If some metric has different docstrings in various jobs, then log a warning and overwrite it (the last write wins).

Obviously, they also should be exposed via API.

[aweiteka]

+1. Here's a specific use case:

As a query developer creating queries for dashboards and alerts, I would like to enable "help mode" in the prometheus web UI so I can understand exactly what data is being returned so I have confidence the data I am getting is what I want. This might be implemented as a mouseover tooltip in the query bar or as a function, e.g. help(<metric_name>)

[gouthamve]

Related: #376

[brian-brazil]

#4183 added this.

[lock]

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