refactor[Counter + Gauge]: uniquely identify metric time series by name and label sets (1/2)

[incertum]

Previously, metrics were identified by their name. This change allows a shared metric name per metric type, with each unique set of labels defining a distinct time series.

Additionally, the internal data structure for a stored metric has been refactored, providing a more robust and programmatic representation.

CC @FranzBusch, @ktoso, @blindspotbounty

Let me know how you'd prefer to proceed. We have two main options:

1. I can split this work into a few smaller, more focused PRs.
2. We can finalize the overall approach in this PR, and I can push the remaining changes here next week.

Fixes #125 opened by @blindspotbounty

[ktoso]

Overall ok, but the names are really confusing. Perhaps: keep Metric a type, drop MetricContainer, doesn't seem to have anything specific it's adding -- just a single field, and then rename "MetricType" to be the Container type?

[incertum]

I have also added the code changes for Gauge and moved the PR out of WIP.

[ktoso]

This could use some more use of generics rather than copying types around 1:1; here's a patch:
0001-make-metric-groups-use-more-generics.patch

Basically:

-    // Note: In order to support Sendable, need to explicitely define the types.
-    private struct CounterWithHelp {
-        var counter: Counter
+    private struct MetricWithHelp<Metric: AnyObject & Sendable>: Sendable {
+        var metric: Metric
         let help: String
     }

-    private struct GaugeWithHelp {
-        var gauge: Gauge
-        let help: String
-    }
-
-    private struct CounterGroup {
-        // A collection of Counter metrics, each with a unique label set, that share the same metric name.
-        // Distinct help strings for the same metric name are permitted, but Prometheus retains only the
-        // most recent one. For an unlabelled Counter, the empty label set is used as the key, and the
-        // collection contains only one entry. Finally, for clarification, the same Counter metric name can
-        // simultaneously be labeled and unlabeled.
-        var countersByLabelSets: [LabelsKey: CounterWithHelp]
-    }
-
-    private struct GaugeGroup {
-        var gaugesByLabelSets: [LabelsKey: GaugeWithHelp]
+    /// A collection of metrics, each with a unique label set, that share the same metric name.
+    /// Distinct help strings for the same metric name are permitted, but Prometheus retains only the
+    /// most recent one. For an unlabelled metric, the empty label set is used as the key, and the
+    /// collection contains only one entry. Finally, for clarification, the same metric name can
+    /// simultaneously be labeled and unlabeled.
+    private struct MetricGroup<Metric: Sendable & AnyObject>: Sendable {
+        var metricsByLabelSets: [LabelsKey: MetricWithHelp<Metric>]
     }

     private enum Metric {
-        case counter(CounterGroup)
-        case gauge(GaugeGroup)
+        case counter(MetricGroup<Counter>)
+        case gauge(MetricGroup<Gauge>)

[ktoso]

minor: i'd move the comment onto the MetricGroup type

[incertum]

ACK I'll do it in the 2/2 PR!

[ktoso]

Looks good to me, thank you.