Merge pull request #34087 from backstage/otel/tracing-service

feat: `TracingService` to match `MetricService`

.changeset/backend-test-utils-tracing-service-mock.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-test-utils': patch
+---
+
+Added a new tracing service mock to be leveraged in tests

.changeset/backend-tracing-and-stuff.md

@@ -0,0 +1,6 @@
+---
+'@backstage/backend-plugin-api': patch
+'@backstage/backend-defaults': patch
+---
+
+Adds an alpha `TracingService` to provide a unified interface for emitting trace spans across Backstage plugins.