Specify allowed characters for Baggage keys and values

CHANGELOG.md

@@ -33,6 +33,8 @@ release.
 
 - Align definition of Baggage with W3C Specification.
   ([#3800](https://github.com/open-telemetry/opentelemetry-specification/pull/3800))
+- Specify allowed characters for Baggage keys and values.
+  ([#3801](https://github.com/open-telemetry/opentelemetry-specification/pull/3801))
 
 ### Traces
 

specification/baggage/api.md

@@ -35,6 +35,27 @@ with _exactly one value_. This is more restrictive than the [W3C Baggage
 Specification, § 3.2.1.1](https://www.w3.org/TR/baggage/#baggage-string)
 which allows duplicate entries for a given name.
 
+Baggage **names** are strings that satisfy the `token` definition from
+[RFC7230, Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6).
+Alpha-numeric names are strongly recommended. Language API SHOULD return
+an error when given a baggage name that is not a `token`.
+
+Baggage **values** are any valid UTF-8 strings. Language API MUST accept
+any valid UTF-8 string as baggage **value** in `Set` and return the same
+value from `Get`.
+
+Language API MUST treat both baggage names and values as case sensitive.
+See also [W3C Baggage -- Case sensitivity of keys](https://github.com/w3c/baggage/blob/main/baggage/HTTP_HEADER_FORMAT_RATIONALE.md#case-sensitivity-of-keys).
+
+Example:
+
+```
+baggage.Set('a', 'B% 💼');
+baggage.Set('A', 'c');
+baggage.Get('a'); // returns "B% 💼"
+baggage.Get('A'); // returns "c"
+```
+
 The Baggage API consists of:
 
 - the `Baggage` as a logical container