Add PullingGauge gauge type

[JanBerktold]

First of all, thank you for this crate! We've been using it in production for a bit and are super happy.

As part of this, we've written a few additional metric types that allow us to instrument our app in an easier fashion. This PR attempts to merge the first of these back to the upstream project. If there is interest, then we'd be happy to send more PRs with other helper types (i.e. we have a MaximumOverIntervalGauge that allows monitoring of quickly changing values).

The metric type that this PR is adding is the PullingGauge. It is useful in scenarios where we want to expose a particular value but it is infeasible to keep the metric up to date all of the time (i.e. it changes a lot in hot code paths). The most common example for us internally is to expose the size of a Vector or similiar collections. In those cases, we create one of these and pass a move || vector.len() as f64 as the lambda.

I've more or less just lifted this out of our internal codebase -- please let me know in case of any changes required to make it fit better in this crate!

[JanBerktold]

@lucab hi! would you mind taking a look at this PR?

[jmorag]

Just wanted to second that this functionality would be very nice to have. It exists in the go client and we miss it in rust.

[anacrolix]

We've been using this in production for a very long time. Please consider for merging!

[JanBerktold]

Hey @lucab, would it be possible for you to take a look here? Thanks!

[JanBerktold]

@lucab Hey there! We'd love to get this upstreamed if possible. Is there anything else I can do to help with this?

[JanBerktold]

Thanks for the comments! I think I've addressed all of them now -- Please take a look, @lucab.

[anacrolix]

Thanks @lucab and @JanBerktold !

[lucab]

Thanks for this, just one last minor nit on the doc example.

After that, it should be ready to merge, feel free to squash all of this into a single commit.

[JanBerktold]

Thanks for this, just one last minor nit on the doc example.

After that, it should be ready to merge, feel free to squash all of this into a single commit.

Should be good to go now! Changed the example & squashed all commits.

[lucab]

LGTM!