Histogram backed timer

[avolokhov]

Hey @MrLotU,

Thanks for considering this PR. In my project I'm running multiple replicas of my backend service. For a proper monitoring I needed aggregatable timer metrics to get cumulative percentiles across the whole fleet. Here's the solution I came up with. Since the project is in alpha, I decided to start with breaking a few APIs in order to keep the code simple. Please let me know if you think we should approach this differently.

Fixes https://github.com/MrLotU/SwiftPrometheus/issues/44

Checklist

• [+] The provided tests still run.
• [+] I've created new tests where needed.
• [+] I've updated the documentation if necessary.

Motivation and Context

Server side applications are never run in as single instance. Summary backed timer doesn't allow aggregation and therefore doesn't allow to render accumulated latency percentiles across multiple instances. Histogram backed timer provides a solution with it. Timer implementation is yet another fact that only makes sense for bridging Prometheus to swift-metrics (in addition to Labels that are init parameters in swift-metrics world and call parameters in Prometheus world, and therefore LabelSanitizer). It feels natural to separate non-trivial swift-metrics bridge from Prometheus core, keeping either of them simpler and more testable.

Description

• MetricsFactory conformance is now a wrapper around PrometheusClient. This allows us to be much more flexible and configurable when bridging swift-metrics api to Prometheus backend (i.e. picking a Timer implementation, setting default quantiles/buckets, in future potentially setting base labels).
• Introduced configuration object.
• Introduced Histogram backed Timer implementation.

[grennis]

+1 for this.

I tried the branch and it seemed to work great. The only problem I saw was that in Vapor using:

        try MetricsSystem.prometheus().collect(into: promise)

gave me Fatal error: leaking promise. But I worked around by using the client I had created:

        client.collect(into: promise)

And this worked.

In any case would love to see this get merged, as providing a histogram backed timer is really a necessity.

[MrLotU]

+1 for this.

I tried the branch and it seemed to work great. The only problem I saw was that in Vapor using:

        try MetricsSystem.prometheus().collect(into: promise)

gave me Fatal error: leaking promise. But I worked around by using the client I had created:

        client.collect(into: promise)

And this worked.

In any case would love to see this get merged, as providing a histogram backed timer is really a necessity.

@grennis The issue you mention about leaking promises seems unrelated to this PR, but should not be happening for sure. Would you mind filing a bug-report with some extra information like system setup, full error message and reproduction steps? You can do so here

Thanks!

[avolokhov]

@MrLotU thanks for the review! I believe, I've addressed all the comments.

[MrLotU]

Released in 1.0.0-alpha.10