The trace and the trace_ppx packages for OCaml allow to instrument a
program by invoking span, message (event) or counter operations,
explicitly by using the functions provided by the former package, or
automatically by using the syntax extension provided by the later.

There exist multiple OCaml libraries that provide a backend compatible
with this package, under others:

- trace-fushia (included): implementing the Fushia project's trace
  format,
- trace-tef (included): simple file output using the TEF/Catapult format,
- tracy-client.trace,
- ocaml-opentelemetry: OpenTelemetry support.