Allow UpDown counters to be 'set'

[RichardWright]

Current spec for UpDownCounters

.inc(5)
.inc(-5)

If the state is based on, for example, the length of an array (eg a list of connected clients), this requires the previous state to be stored somewhere so that subtraction can happen. 

Proposal:

.set(25)

Allow the instrument to be set, this can be then used to map to properties of data structures which inform this and remove the need for diffing. This is possible with an observer but using this required substantial changes to our app and exposing state where we don’t want to. With a synchronous set method, this can be done within methods without exposing internal state to observers. 

The Nodejs package ‘prom-client’ has this functionality already and we find it very useful. 

[imgalli]

+1 on this

[jmacd]

This request is on par with the request for a synchronous Gauge instrument, which I support.

As far as #2226 is concerned, we want the UpDownCounter that is Set() to be different than one that that uses. Add()

As far as how the specification is written today, it's easier to defined this as "Observing an asynchronous instrument synchronously", since it has the same metric identity as the asynchronous UpDownCounter. All we need to do is to allow these instruments (i.e., UpDownCounter, Gauge) to support Observe() outside of the asynchronous callbacks.