Specify allowed characters for Baggage keys and values (#3801)

## Changes

* Add missing spec language on what keys and values are allowed for the
baggage.
* Emphasize case sensitivity
* Definitions are aligned with W3C Baggage Spec, but provide a
distinction between API representation and the wire encoding (done by
W3C Propagator)

Author: yurishkuro

CHANGELOG.md

@@ -91,6 +91,8 @@ 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 +35,32 @@ 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 any valid UTF-8 strings. Language API SHOULD NOT
+restrict which strings are used as baggage **names**. However, the
+specific `Propagator`s that are used to transmit baggage entries across
+component boundaries may impose their own restrictions on baggage names.
+For example, the [W3C Baggage specification](https://www.w3.org/TR/baggage/#key)
+restricts the baggage keys to strings that satisfy the `token` definition
+from [RFC7230, Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6).
+For maximum compatibility, alpha-numeric names are strongly recommended
+to be used as baggage names.
+
+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 Rationale](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