Merge internal otel distribution spec (#865)

* fix typo

* merge internal spec elements

* Update specs/agents/otel-distribution.md

Co-authored-by: Steve Gordon <sgordon@hotmail.co.uk>

* Update specs/agents/otel-distribution.md

Co-authored-by: Christos Kalkanis <christos.kalkanis@elastic.co>

---------

Co-authored-by: Steve Gordon <sgordon@hotmail.co.uk>
Co-authored-by: Christos Kalkanis <christos.kalkanis@elastic.co>

specs/agents/mobile/README.md

@@ -7,7 +7,8 @@ The mobile specific docs can be found:
 * [iOS Agent](https://www.elastic.co/guide/en/apm/agent/swift/0.x/intro.html)
 * [Android Agent](https://www.elastic.co/guide/en/apm/agent/android/current/index.html)
 
-## Open Telementry
+## OpenTelemetry
+
 The mobile agents are the first agents at Elastic to be built on top of Open-Telementry. 
 A large portion of the mobile agents' functionality can be attributed to the [opentelementry-swift](https://github.com/open-telemetry/opentelemetry-swift) and [opentelementry-java](https://github.com/open-telemtry/opentelemetry-java) packages.
 
@@ -16,7 +17,6 @@ However, the Elastic mobile agents don't set every attribute defined (many only
 
 ## Semantic Conventions and APM Server Mappings
 
-
 ### Resource Attributes 
 Here is a list of resource attributes that are relevant for our mobile agents:
 

specs/agents/otel-distribution.md

@@ -36,6 +36,51 @@ When introducing new features, the decision between starting with platform-speci
 For simplicity the configuration in this specification will use the "environment variable" syntax, some platforms like Java
 might also support other ways to configure.
 
+## Identification
+
+### User Agent headers
+
+Per the [OpenTelemetry Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#user-agent), OpenTelemetry SDKs are expected to send a `User-Agent` header when exporting data to a backend. At a minimum, this header SHOULD identify the exporter, the language of its implementation, and the version of the exporter.
+
+Elastic distributions SHOULD configure a customized `User-Agent` header when possible[^1].
+This allows data exported from a vanilla SDK and an Elastic distribution to be easily distinguished.
+
+[^1]: Some OpenTelemetry SDKs (e.g. .NET) do not provide a mechanism to modify the `User-Agent` header. In this case, we accept their default.
+
+To conform with [RFC7231](https://datatracker.ietf.org/doc/html/rfc7231#section-5.5.3), the existing SDK `User-Agent` should be preceded by a product identifier and version for the Elastic distribution.
+
+```
+<repo-name>/<version> <existing-otel-sdk-user-agent>
+```
+
+For example, in the .NET distribution, the `User-Agent` header would be configured as follows:
+
+```
+elastic-otel-dotnet/1.0.0 OTel-OTLP-Exporter-Dotnet/1.6.0
+```
+
+### Telemetry resource attributes
+
+Per the [semantic conventions](https://opentelemetry.io/docs/specs/semconv/resource/#telemetry-sdk), SDKs are expected to include the following resource attributes on captured signals. These are used to identify the SDK where the data was captured and should not be modified.
+
+- `telemetry.sdk.name`
+- `telemetry.sdk.version`
+- `telemetry.sdk.language`
+
+In the above attributes, the name and version should be the OTel SDK name and version.
+The language should be the primary language that the SDK is intended for.
+It is expected that the OpenTelemetry SDK sets these values.
+Our distros should set them, only if the SDK code does not do so automatically.
+
+Intake currently reads these attributes and uses them to populate the `agent.Name` and `agent.Version` fields.
+
+The semantic conventions also [define two experimental attributes](https://opentelemetry.io/docs/specs/semconv/resource/#telemetry-sdk-experimental) to identify the distribution:
+
+- `telemetry.distro.name`: must be set to `elastic`
+- `telemetry.distro.version`: must reflect the distribution version
+
+Distributions SHOULD set these attributes with appropriate values.
+
 ## Features
 
 ### Inferred spans
@@ -79,4 +124,4 @@ Implementation is currently platform specific:
 
 Supported platforms: [Java](https://github.com/elastic/elastic-otel-java/tree/main/universal-profiling-integration)
 
-For the configuration options see [this section](universal-profiling-integration.md#configuration-options).
+For the configuration options see [this section](universal-profiling-integration.md#configuration-options).