feat(telemetry): add configurable histogram buckets for metrics (#3098)

You can customize the buckets for all generated histograms:

```yaml title="router.yaml"
telemetry:
  metrics:
    common:
      buckets:
        - 0.05
        - 0.10
        - 0.25
        - 0.50
        - 1.00
        - 2.50
        - 5.00
        - 10.00
        - 20.00
```

Fixes #2333

**Checklist**

Complete the checklist (and note appropriate exceptions) before a final
PR is raised.

- [x] Changes are compatible[^1]
- [x] Documentation[^2] completed
- [ ] Performance impact assessed and acceptable
- Tests added and passing[^3]
    - [x] Unit Tests
    - [ ] Integration Tests
    - [x] Manual Tests

**Exceptions**

*Note any exceptions here*

**Notes**

[^1]. It may be appropriate to bring upcoming changes to the attention
of other (impacted) groups. Please endeavour to do this before seeking
PR approval. The mechanism for doing this will vary considerably, so use
your judgement as to how and when to do this.
[^2]. Configuration is an important part of many changes. Where
applicable please try to document configuration examples.
[^3]. Tick whichever testing boxes are applicable. If you are adding
Manual Tests:
- please document the manual testing (extensively) in the Exceptions.
- please raise a separate issue to automate the test and label it (or
ask for it to be labeled) as `manual test`

---------

Signed-off-by: Benjamin Coenen <5719034+bnjjj@users.noreply.github.com>

.changesets/feat_swan_cub_foot_audience.md

@@ -0,0 +1,21 @@
+### Configurable histogram buckets for metrics ([Issue #2333](https://github.com/apollographql/router/issues/2333))
+
+You can customize the buckets for all generated histograms:
+
+```yaml title="router.yaml"
+telemetry:
+  metrics:
+    common:
+      buckets:
+        - 0.05
+        - 0.10
+        - 0.25
+        - 0.50
+        - 1.00
+        - 2.50
+        - 5.00
+        - 10.00
+        - 20.00
+```
+
+By [@bnjjj](https://github.com/bnjjj) in https://github.com/apollographql/router/pull/3098

apollo-router/src/configuration/snapshots/apollo_router__configuration__tests__schema_generation.snap

@@ -3711,6 +3711,28 @@ expression: "&schema"
                   "additionalProperties": false,
                   "nullable": true
                 },
+                "buckets": {
+                  "description": "Custom buckets for histograms",
+                  "default": [
+                    0.001,
+                    0.005,
+                    0.015,
+                    0.05,
+                    0.1,
+                    0.2,
+                    0.3,
+                    0.4,
+                    0.5,
+                    1.0,
+                    5.0,
+                    10.0
+                  ],
+                  "type": "array",
+                  "items": {
+                    "type": "number",
+                    "format": "double"
+                  }
+                },
                 "resources": {
                   "description": "Resources",
                   "default": {},
@@ -3721,11 +3743,13 @@ expression: "&schema"
                 },
                 "service_name": {
                   "description": "Set a service.name resource in your metrics",
+                  "default": null,
                   "type": "string",
                   "nullable": true
                 },
                 "service_namespace": {
                   "description": "Set a service.namespace attribute in your metrics",
+                  "default": null,
                   "type": "string",
                   "nullable": true
                 }

apollo-router/src/plugins/telemetry/config.rs

@@ -80,17 +80,25 @@ pub(crate) struct Metrics {
 }
 
 #[derive(Clone, Default, Debug, Deserialize, JsonSchema)]
-#[serde(deny_unknown_fields, rename_all = "snake_case")]
+#[serde(deny_unknown_fields, rename_all = "snake_case", default)]
 pub(crate) struct MetricsCommon {
     /// Configuration to add custom labels/attributes to metrics
     pub(crate) attributes: Option<MetricsAttributesConf>,
     /// Set a service.name resource in your metrics
     pub(crate) service_name: Option<String>,
     /// Set a service.namespace attribute in your metrics
     pub(crate) service_namespace: Option<String>,
-    #[serde(default)]
     /// Resources
     pub(crate) resources: HashMap<String, String>,
+    /// Custom buckets for histograms
+    #[serde(default = "default_buckets")]
+    pub(crate) buckets: Vec<f64>,
+}
+
+fn default_buckets() -> Vec<f64> {
+    vec![
+        0.001, 0.005, 0.015, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 5.0, 10.0,
+    ]
 }
 
 /// Tracing configuration

apollo-router/src/plugins/telemetry/metrics/otlp.rs

@@ -42,9 +42,7 @@ impl MetricsConfigurator for super::super::otlp::Config {
             Some(exporter) => {
                 let exporter = opentelemetry_otlp::new_pipeline()
                     .metrics(
-                        selectors::simple::histogram([
-                            0.001, 0.005, 0.015, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 5.0, 10.0,
-                        ]),
+                        selectors::simple::histogram(metrics_config.buckets.clone()),
                         aggregation::stateless_temporality_selector(),
                         opentelemetry::runtime::Tokio,
                     )

apollo-router/src/plugins/telemetry/metrics/prometheus.rs

@@ -85,9 +85,7 @@ impl MetricsConfigurator for Config {
         if self.enabled {
             let mut controller = controllers::basic(
                 processors::factory(
-                    selectors::simple::histogram([
-                        0.001, 0.005, 0.015, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 5.0, 10.0,
-                    ]),
+                    selectors::simple::histogram(metrics_config.buckets.clone()),
                     aggregation::stateless_temporality_selector(),
                 )
                 .with_memory(true),