Add a changelog file

CHANGELOG.md

@@ -0,0 +1,27 @@
+# Changelog
+
+## Unreleased
+
+### API
+
+#### Context
+
+- Propagators must now be implementations of a propagator type's behaviour. At
+  this time only the `otel_propagator_text_map` behaviour exists. Callbacks for
+  inject and extract take an optional "set" and "get" function for working with
+  a carrier.
+- Configuration of propagators is now a list of atoms representing either the
+  name of a builtin propagator (at this time those are, `trace_context`, `b3` and
+  `baggage`) or the name of a module implementing the propagator's behaviour.
+  - Default configuration: `{text_map_propagators, [trace_context, baggage]}`
+- Injectors and extractors can be configured separately instead of using the
+  same list of propagators for both by configuring `text_map_injectors` and
+  `text_map_extractors`.
+  - For example you may want your service to support receiving `b3` headers
+    but have no need for it including `b3` headers when it is propagating to
+    other services:
+
+    ```
+    {text_map_injectors, [trace_context, baggage]},
+    {text_map_extractors, [b3, trace_context, baggage]}
+    ```