docs: add metrics layer attributes description (#49)

## Motivation

* fix broken link in doc
* update histogram metrics field prefix (`value` -> `histogram`)
* add metrics layer attributes description
  #43

Author: ymgyt

src/lib.rs

@@ -27,7 +27,7 @@
 //! * `otel.status_message`: Set the span status message.
 //!
 //! [span kinds]: opentelemetry::trace::SpanKind
-//! [span status codes]: opentelemetry::trace::StatusCode
+//! [span status codes]: opentelemetry::trace::Status
 //!
 //! ### Semantic Conventions
 //!
@@ -82,11 +82,13 @@
 //!
 //! ## Feature Flags
 //!
-//! - `metrics`: Enables the [`MetricsSubscriber`] type, a [subscriber] that
+//! - `metrics`: Enables the [`MetricsLayer`] type, a [layer] that
 //!   exports OpenTelemetry metrics from specifically-named events. This enables
 //!   the `metrics` feature flag on the `opentelemetry` crate.  *Enabled by
 //!   default*.
 //!
+//! [layer]: tracing_subscriber::layer
+//!
 //! ## Supported Rust Versions
 //!
 //! Tracing is built against the latest stable release. The minimum supported

src/metrics.rs

@@ -247,8 +247,7 @@ impl<'a> Visit for MetricVisitor<'a> {
 /// - `monotonic_counter.` (non-negative numbers): Used when the counter should
 ///   only ever increase
 /// - `counter.`: Used when the counter can go up or down
-/// - `value.`: Used for discrete data points (i.e., summing them does not make
-///   semantic sense)
+/// - `histogram.`: Used to report arbitrary values that are likely to be statistically meaningful
 ///
 /// Examples:
 /// ```
@@ -260,9 +259,9 @@ impl<'a> Visit for MetricVisitor<'a> {
 /// info!(counter.baz = -1);
 /// info!(counter.xyz = 1.1);
 ///
-/// info!(value.qux = 1);
-/// info!(value.abc = -1);
-/// info!(value.def = 1.1);
+/// info!(histogram.qux = 1);
+/// info!(histogram.abc = -1);
+/// info!(histogram.def = 1.1);
 /// ```
 ///
 /// # Mixing data types
@@ -307,6 +306,19 @@ impl<'a> Visit for MetricVisitor<'a> {
 /// info!(counter.baz = (i64::MAX as u64) + 1)
 /// ```
 ///
+/// # Attributes
+///
+/// When `MetricsLayer` outputs metrics, it converts key-value pairs into [Attributes] and associates them with metrics.
+///
+/// [Attributes]: https://opentelemetry.io/docs/specs/otel/common/#attribute
+///
+/// For example:
+/// ```
+/// # use tracing::info;
+/// // adds attributes bar="baz" and qux=2 to the `foo` counter.
+/// info!(monotonic_counter.foo = 1, bar = "baz", qux = 2);
+/// ```      
+///
 /// # Implementation Details
 ///
 /// `MetricsLayer` holds a set of maps, with each map corresponding to a