New Version 2.0 API

[fabianfett]

Motivation

Swift Prometheus has been in development since 2018. The previous maintainer (@MrLotU) stepped down about a year ago. Since then @ktoso and @fabianfett have maintained the existing code base.

In order to improve performance in the Prometheus library, we want to replace the existing implementation with a new one for version 2.0. While we do this we also want to get the general API closer to what the prometheus project recommends in their Writing client libraries guide. This means that we will also start to enforce Prometheus rules. For example having two counter with the same name, but one with labels and one without, is illegal in the Prometheus format. Lastly we want to remove the SwiftNIO dependency that is not needed anymore.

I'm well aware that this is a very large PR that changes lots of moving things.

Changes

• Remove SwiftNIO dependency. Vendor in NIOLock from NIOConcurrencyHelpers
• Replace PrometheusClient with PrometheusCollectorRegistry
• Remove Summary implementation
• Replace Counter, Gauge and Histogram implementations
• Add Sendability annotations
• Require Swift 5.7
• Enforces that label names remain constant for a metric name.

Performance difference

In simple testing scenarios:

• Exporting values from 1k counters into the prometheus format: ~10x speedup
• Exporting values from 1k gauges into the prometheus format: ~10x speedup
• Exporting values from 1k histograms into the prometheus format: ~25x speedup

Differences to Writing client libraries guide

• There is no explicit Collector protocol, that users could implement to create their own instruments.
  • Counter, Gauge and Histogram should implement Collector
• Currently the PrometheusCollectorRegistry does not offer a register and unregister method for Collectors
• There is no API to create a Counter/Gauge/Histogram that is not registered with a PrometheusCollectorRegistry
• Collectors can't be registered with multiple different PrometheusCollectorRegistrys
• We don't advertise a default collector

Do we want to offer this API? With the current patch we could add this API later.

[Andrewangeta]

Looks nice! :shipit:

[ktoso]

Feel free to merge at will btw 👍