Update to otel 0.20.0

.changesets/maint_bryn_otel_update.md

@@ -0,0 +1,22 @@
+### Update to OpenTelemetry 0.20.0 ([PR #3649](https://github.com/apollographql/router/pull/3649))
+
+The router now uses OpenTelemetry 0.20.0. This includes a number of fixes and improvements from upstream.
+
+In particular metrics have some significant changes:
+* Prometheus metrics are now aligned with the [OpenTelemetry spec](https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/), and will not report `service_name` on each individual metric. Resource attributes are now moved to a single `target_info` metric.
+
+  Users should check that their dashboards and alerts are properly configured when upgrading.
+
+* The default service name for metrics is now `unknown_service` as per the [OpenTelemetry spec](https://opentelemetry.io/docs/concepts/sdk-configuration/general-sdk-configuration/#otel_service_name).
+
+  Users should ensure to configure service name via router.yaml, or via the `OTEL_SERVICE_NAME` environment variable. 
+
+  We will be following bring tracing into alignment in future. 
+
+* The order of priority for setting service name has been brought into line with the rest of the router configuration. The order of priority is now:
+  1. `OTEL_RESOURCE_ATTRIBUTES` environment variable
+  2. `OTEL_SERVICE_NAME` environment variable
+  3. `resource_attributes` in router.yaml
+  4. `service_name` in router.yaml
+
+By [@BrynCooke](https://github.com/BrynCooke) in https://github.com/apollographql/router/pull/3649