Propagation - Extract handles multiple values on carrier using same key

CHANGELOG.md

@@ -9,6 +9,9 @@ release.
 
 ### Context
 
+- Propagation - Extract handles multiple values on carrier using same key.
+  [#4295](https://github.com/open-telemetry/opentelemetry-specification/pull/4295)
+
 ### Traces
 
 ### Metrics

specification/context/api-propagators.md

@@ -22,6 +22,7 @@
     + [Getter argument](#getter-argument)
       - [Keys](#keys)
       - [Get](#get)
+      - [GetAll](#getall)
 - [Injectors and Extractors as Separate Interfaces](#injectors-and-extractors-as-separate-interfaces)
 - [Composite Propagator](#composite-propagator)
   * [Create a Composite Propagator](#create-a-composite-propagator)
@@ -196,11 +197,11 @@ Returns a new `Context` derived from the `Context` passed as argument.
 
 #### Getter argument
 
-Getter is an argument in `Extract` that get value from given field
+Getter is an argument in `Extract` that gets value(s) from given field.
 
 `Getter` allows a `TextMapPropagator` to read propagated fields from a carrier.
 
-One of the ways to implement it is `Getter` class with `Get` and `Keys` methods
+One of the ways to implement it is `Getter` class with methods `Get`, `Keys`, and `GetAll`
 as described below. Languages may decide on alternative implementations and
 expose corresponding methods as delegates or other ways.
 
@@ -229,6 +230,24 @@ Required arguments:
 
 The Get function is responsible for handling case sensitivity. If the getter is intended to work with a HTTP request object, the getter MUST be case insensitive.
 
+##### GetAll
+
+The GetAll function MUST NOT require explicit implementation to satisfy the base `Getter` type.
+Language implementations have the flexibility to incorporate this optional
+functionality in various ways, such as by providing a default GetAll method, or
+by creating a new type that extends the base `Getter` type.
+
+If explicitly implemented, the GetAll function MUST return all values of the given propagation key.
+It SHOULD return them in the same order as they appear in the carrier.
+If the key doesn't exist, it SHOULD return an empty collection.
+
+Required arguments:
+
+- the carrier of propagation fields, such as an HTTP request.
+- the key of the field.
+
+The GetAll function is responsible for handling case sensitivity. If the getter is intended to work with a HTTP request object, the getter MUST be case insensitive.
+
 ## Injectors and Extractors as Separate Interfaces
 
 Languages can choose to implement a `Propagator` type as a single object