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22d09f62bc
In the upstream `opentelemetry` crate, the `trace` and `metrics` features are gated by separate feature flags. This allows users who are only using OpenTelemetry for tracing, or who are only using it for metrics, to pick and choose what they depend on. Currently, the release version of `tracing-opentelemetry` only provides tracing functionality, and therefore, it only depends on `opentelemetry` with the `trace` feature enabled. However, the metrics support added in #2185 adds a dependency on the `opentelemetry/metrics` feature. This is currently always enabled. We should probably follow the same approach as upstream `opentelemetry`, and allow enabling/disabling metrics and tracing separately. This branch adds a `metrics` feature to `tracing-opentelemetry`, and makes the `MetricsLayer` from #2185 gated on the `metrics` feature. This feature flag is on by default, like the upstream `opentelemetry/metrics` feature, but it can be disabled using `default-features = false`. We should probably do something similar for the tracing components of the crate, and make them gated on a `trace` feature flag, but adding a feature flag to released APIs is not semver-compatible, so we should save that until the next breaking release.