message: opentelemetry: Add semantic convention docs
  This branch adds crate-level docs for the otel trace semantic
  conventions and their usage as well as notes on stability status.
diff: //! Setting this field is useful if you want to display non-static information
  //! in your span name.
  //!
  +//! ### Semantic Conventions
  +//!
  +//! OpenTelemetry defines conventional names for attributes of common
  +//! operations. These names can be assigned directly as fields, e.g.
  +//! `trace_span!("request", "span.kind" = "client", "http.url" = ..)`, and they
  +//! will be passed through to your configured OpenTelemetry exporter. You can
  +//! find the full list of the operations and their expected field names in the
  +//! [semantic conventions] spec.
  +//!
  +//! [semantic conventions]: https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/trace/semantic_conventions
  +//!
  +//! ### Stability Status
  +//!
  +//! The OpenTelemetry specification is currently in beta so some breaking
  +//! may still occur on the path to 1.0. You can follow the changes via the
  +//! [spec repository] to track progress toward stabilization.
  +//!
  +//! [spec repository]: (https://github.com/open-telemetry/opentelemetry-specification)
  +//!
  //! ## Examples
  //!
  //! ```
  