Add prometheus-lwt as a forward-compatible Lwt interface

[avsm]

Introduce the prometheus-lwt package with the API that the core will keep once its Lwt dependency is removed (#60 #65), but implemented for now via Lwt support still present in prometheus.

Therefore nothing breaks in this release as existing users can migrate to Prometheus_lwt and also to the new synchronous _fn functions at their own pace. The actual interface break will then happen in a later release without further source changes for migrated Lwt users.

Also add synchronous variants of the timing helpers to the core since the existing Lwt-typed functions occupy the unsuffixed names until the break.

Suggested by @talex5 in #65.

[talex5]

I've pushed a commit with some changes:

We'd like people to stop using the old names soon so we can remove the Lwt dependency, so I marked the old functions as deprecated now.

API changes:

• Remove the _lwt suffix from functions in Prometheus_lwt as redundant (an alternative would be to include the main API here, in which case keeping the suffix would make sense, but I think it's better to avoid using the Lwt package when you don't need it).

• The new name for Gauge.time is now set_time and it sets the time instead of incrementing it (this is what it was supposed to do; see https://prometheus.io/docs/instrumenting/writing_clientlibs/).

• The replacement for track_inprogress is now track_in_progress.

• Summary.time becomes Summary.observe_time, and the same for histograms.

This should mean we end up with a nicer API after removing the old names I think.

All functions in prometheus.mli that mention Lwt are now deprecated, except for collect (which isn't used by libraries, so doesn't matter).

[avsm]

Looks good, I was in two minds about the deprecated tags since I really dislike build time spam, but these are all actionable tags with instructions on what to do for the user!

[talex5]

Looks good to me.

I pushed some more minor changes, updating the changelog and README a bit, and removing the (wrapped false) (didn't look like it was needed).

[avsm]

This is a minor thing, but is the deprecated tag needed outside of the mli?

[talex5]

It wouldn't build without it for some reason (the error is pretty confusing):

File "src/prometheus.ml", line 1:   
Error (alert deprecated): time
Use observe_time instead if you don't need Lwt here, or Prometheus_lwt.HISTOGRAM.observe_time if you do.
File "src/prometheus.mli", lines 233-234, characters 2-123:
  Definition
File "src/prometheus.ml", line 367, characters 2-67:
367 |   val time : t -> (unit -> float) -> (unit -> 'a Lwt.t) -> 'a Lwt.t
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  Expected signature

[avsm]

Interesting, it doesn't fire an error when using let bindings and val signatures, only when module types are involved.