feat(exporters): add user-agent enrichment (#4560)

Fixes #4479

## Changes

Document that exporters may allow a `user-agent` option to append a
product identifier to the exporter User Agent header. such option is
only available via local var (not environment)

For non-trivial changes, follow the [change proposal
process](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CONTRIBUTING.md#proposing-a-change).

* [ ] Related issues #
* [ ] Related [OTEP(s)](https://github.com/open-telemetry/oteps) #
* [ ] Links to the prototypes (when adding or changing features)
* [ ]
[`CHANGELOG.md`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CHANGELOG.md)
file updated for non-trivial changes
* [ ]
[`spec-compliance-matrix.md`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md)
updated if necessary

---------

Co-authored-by: Trent Mick <[email protected]>
Co-authored-by: Cijo Thomas <[email protected]>
Co-authored-by: Riccardo Magliocchetti <[email protected]>

Author: 4 people

CHANGELOG.md

@@ -31,6 +31,8 @@ release.
 
 ### Common
 
+- OTLP Exporters may allow devs to prepend a product identifier in `User-Agent` header.
+  ([#4560](https://github.com/open-telemetry/opentelemetry-specification/pull/4560))
 - ⚠️ **IMPORTANT**: Extending the set of standard attribute value types is no longer a breaking change.
   ([#4614](https://github.com/open-telemetry/opentelemetry-specification/pull/4614))
 

specification/protocol/exporter.md

@@ -210,6 +210,12 @@ OTel-OTLP-Exporter-Python/1.2.3
 
 The format of the header SHOULD follow [RFC 7231][rfc-7231]. The conventions used for specifying the OpenTelemetry SDK language and version are available in the [Resource semantic conventions][resource-semconv].
 
+Exporters MAY expose a configuration option to add a product identifier to the User-Agent header. The resulting User-Agent SHOULD include the exporter's default User-Agent string. The intention is to support an identifier for an OpenTelemetry SDK/Agent [distribution][opentelemetry-distribution]. Typically an exporter would *prepend* the given identifier to its own. For example:
+
+```
+MyDistribution/x.y.z OTel-OTLP-Exporter-Python/1.2.3
+```
+
 [Boolean]: ../configuration/sdk-environment-variables.md#boolean
 [Timeout]: ../configuration/sdk-environment-variables.md#timeout
 [String]: ../configuration/sdk-environment-variables.md#string
@@ -223,3 +229,4 @@ The format of the header SHOULD follow [RFC 7231][rfc-7231]. The conventions use
 [otlp-http]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#otlphttp
 [retryable-grpc-status-codes]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#failures
 [retryable-http-status-codes]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#failures-1
+[opentelemetry-distribution]: https://opentelemetry.io/docs/concepts/distributions/