Document how to use labels (#101)

Co-authored-by: hamzahrmalik <[email protected]>

Author: fabianfett

Sources/Prometheus/Docs.docc/index.md

@@ -9,6 +9,8 @@ values in the Prometheus text format.
 
 ``Prometheus`` integrates with [Swift Metrics](doc:swift-metrics).
 
+For general advice on how to use `Prometheus` make sure to also read the [Prometheus documentation][prometheus-docs].
+
 ## Installation
 
 ``Prometheus`` is available through Swift Package Manager. To include it in your project add the 
@@ -68,6 +70,7 @@ print(String(decoding: buffer, as: Unicode.UTF8.self))
 ### Getting started
 
 - <doc:swift-metrics>
+- <doc:labels>
 - ``PrometheusCollectorRegistry``
 - ``PrometheusMetricsFactory``
 
@@ -79,3 +82,5 @@ print(String(decoding: buffer, as: Unicode.UTF8.self))
 - ``Histogram``
 - ``DurationHistogram``
 - ``ValueHistogram``
+
+[prometheus-docs]: https://prometheus.io/docs/introduction/overview/

Sources/Prometheus/Docs.docc/labels.md

@@ -0,0 +1,84 @@
+# Use Labels in Swift Prometheus
+
+Learn how to use Labels in Prometheus, the benefits of doing so, and how to avoid common mistakes.
+
+## Overview
+
+Prometheus Collectors have a name and they may have labels. Labels specify the metrics further. For
+example you might have a ``Counter`` with the name `http_responses_total`. You may now add a label
+`code` that specifies the http status response code. This way you are able to query how many http
+responses were sent. But you can also filter this by response code.
+
+Read more about the benefits of using Labels in the [Prometheus best practices documentation][prometheus-use-labels].
+
+> Note: The naming between Prometheus and Swift-Metrics is a bit confusing. Swift Metrics calls a 
+> metric's name its label and they call a metric's labels dimensions. In this article, when we 
+> refer to labels, we mean the additional properties that can be added to a metrics name.
+>
+> | Framework     | Metric name | Additional infos |
+> |---------------|-------------|------------------|
+> | swift-metrics | `label`     | `dimensions`     |
+> | Prometheus    | `name`      | `labels`         |
+
+Please be aware that the ``PrometheusCollectorRegistry`` will create a seperate metric for each 
+unique label pair, even though the metric name might be the same. This means that in the example 
+below, we will have two independent metrics: 
+
+```swift
+let counter200 = registry.makeCounter(name: "http_responses_total", labels: ["code": "200"])
+let counter400 = registry.makeCounter(name: "http_responses_total", labels: ["code": "400"])
+
+// handling response code
+swift responseCode {
+case .ok:
+  counter200.increment()
+case .badRequest:
+  counter400.increment()
+default:
+  break
+}
+```
+
+> Important: Please note, that all metrics with the same name, **must** use the same label names. 
+
+Prometheus requires that for the same metric name all labels names must be the same. Swift 
+Prometheus enforces this by crashing if the label names or the metric type does not match a 
+previously registered metric with the same name.
+
+#### Examples:
+
+The example below crashes as we try to create a ``Counter`` named `"http_responses_total"` with a 
+label `"code"` after a ``Counter`` with the same name without labels was created earlier.
+
+```swift
+let counter = registry.makeCounter(name: "http_responses_total")
+let counter200 = registry.makeCounter( // 💥 crash
+  name: "http_responses_total", 
+  labels: ["code": "200"]
+)
+```
+
+The example below crashes as we try to create a ``Counter`` named `"http_responses_total"` with a 
+label `"version"` after a ``Counter`` with the same name but different label name `"code"` was 
+created earlier.
+
+```swift
+let counter200 = registry.makeCounter(
+  name: "http_responses_total",
+  labels: ["code": "200"]
+)
+let counterHTTPVersion1 = registry.makeCounter( // 💥 crash
+  name: "http_responses_total", 
+  labels: ["version": "1.1"]
+)
+```
+
+The example below crashes as we try to create a ``Gauge`` named `http_responses_total` with the 
+same name as a previously created ``Counter``.
+
+```swift
+let counter = registry.makeCounter(name: "http_responses_total")
+let gauge = registry.makeGauge(name: "http_responses_total") // 💥 crash
+```
+
+[prometheus-use-labels]: https://prometheus.io/docs/practices/instrumentation/#use-labels

Sources/Prometheus/Docs.docc/swift-metrics.md

@@ -1,7 +1,7 @@
-# Usage with SwiftMetrics
+# Emit metrics collected by Swift Metrics
 
-In this article you will learn how Swift Prometheus integrates with Swift Metrics and how you can
-use both libraries together.
+Learn how Swift Prometheus integrates with Swift Metrics – an abstract API that is widely used in 
+the swift-server ecosystem.
 
 ## Overview
 
@@ -102,6 +102,11 @@ Metrics.Counter(label: "my_counter") // will show up in Prometheus exports as `c
 This can be particularly usefull, if you want to change the names and labels for metrics that are
 generated in a third party library.
 
+> Important: Please note, that all Prometheus metrics with the same name, **must** use the same 
+> label names.
+> Use the ``PrometheusMetricsFactory/nameAndLabelSanitizer`` to ensure this remains true metrics 
+> that are created in third party libraries. See <doc:labels> for more information about this.
+
 ### Defining Buckets for Histograms
 
 #### Default buckets