opentelemetry: feature-flag MetricsSubscriber

tracing-opentelemetry/Cargo.toml

@@ -20,10 +20,12 @@ edition = "2018"
 rust-version = "1.46.0"
 
 [features]
-default = ["tracing-log"]
+default = ["tracing-log", "metrics"]
+# Enables support for exporting OpenTelemetry metrics
+metrics = ["opentelemetry/metrics"]
 
 [dependencies]
-opentelemetry = { version = "0.17.0", default-features = false, features = ["trace", "metrics"] }
+opentelemetry = { version = "0.17.0", default-features = false, features = ["trace"] }
 tracing = { path = "../tracing", version = "0.2", default-features = false, features = ["std"] }
 tracing-core = { path = "../tracing-core", version = "0.2" }
 tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", default-features = false, features = ["registry", "std"] }
@@ -47,3 +49,7 @@ bench = false
 [[bench]]
 name = "trace"
 harness = false
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]

tracing-opentelemetry/README.md

@@ -101,6 +101,12 @@ $ firefox http://localhost:16686/
 
 ![Jaeger UI](trace.png)
 
+## Feature Flags
+
+ - `metrics`: Enables the [`MetricsSubscriber`] type, a [subscriber] that
+   exports OpenTelemetry metrics from specifically-named events. This enables
+   the `metrics` feature flag on the `opentelemetry` crate.
+
 ## Supported Rust Versions
 
 Tracing Opentelemetry is built against the latest stable release. The minimum

tracing-opentelemetry/src/lib.rs

@@ -76,6 +76,13 @@
 //! });
 //! ```
 //!
+//! ## Feature Flags
+//!
+//! - `metrics`: Enables the [`MetricsSubscriber`] type, a [subscriber] that
+//!   exports OpenTelemetry metrics from specifically-named events. This enables
+//!   the `metrics` feature flag on the `opentelemetry` crate.  *Enabled by
+//!   default*.
+//!
 //! ## Supported Rust Versions
 //!
 //! Tracing is built against the latest stable release. The minimum supported
@@ -90,6 +97,7 @@
 //! supported compiler version is not considered a semver breaking change as
 //! long as doing so complies with this policy.
 //!
+//! [subscriber]: tracing_subscriber::subscribe
 #![deny(unreachable_pub)]
 #![cfg_attr(test, deny(warnings))]
 #![doc(html_root_url = "https://docs.rs/tracing-opentelemetry/0.16.0")]
@@ -98,8 +106,18 @@
     html_favicon_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/favicon.ico",
     issue_tracker_base_url = "https://github.com/tokio-rs/tracing/issues/"
 )]
+#![cfg_attr(
+    docsrs,
+    // Allows displaying cfgs/feature flags in the documentation.
+    feature(doc_cfg, doc_auto_cfg),
+    // Allows adding traits to RustDoc's list of "notable traits"
+    feature(doc_notable_trait),
+    // Fail the docs build if any intra-docs links are broken
+    deny(rustdoc::broken_intra_doc_links),
+)]
 
 /// Implementation of the trace::Subscriber trait; publishes OpenTelemetry metrics.
+#[cfg(feature = "metrics")]
 mod metrics;
 /// Span extension which enables OpenTelemetry context management.
 mod span_ext;
@@ -108,6 +126,7 @@ mod subscriber;
 /// Protocols for OpenTelemetry Tracers that are compatible with Tracing
 mod tracer;
 
+#[cfg(feature = "metrics")]
 pub use metrics::MetricsSubscriber;
 pub use span_ext::OpenTelemetrySpanExt;
 pub use subscriber::{subscriber, OpenTelemetrySubscriber};

tracing-opentelemetry/src/metrics.rs

@@ -320,6 +320,7 @@ impl<'a> Visit for MetricVisitor<'a> {
 ///
 /// In the future, this can be improved by associating each `Metric` instance to
 /// its callsite, eliminating the need for any maps.
+#[cfg_attr(docsrs, doc(cfg(feature = "metrics")))]
 pub struct MetricsSubscriber {
     meter: Meter,
     instruments: Instruments,

tracing-opentelemetry/tests/metrics_publishing.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "metrics")]
 use async_trait::async_trait;
 use futures_util::{Stream, StreamExt as _};
 use opentelemetry::{