Add semantic conventions for container id (#673)

* Add semantic conventions for container id

* Update specification/resource/semantic_conventions/README.md

Co-authored-by: Armin Ruech <armin.ruech@dynatrace.com>

* Update specification/resource/semantic_conventions/README.md

Co-authored-by: Armin Ruech <armin.ruech@dynatrace.com>

Co-authored-by: Armin Ruech <armin.ruech@dynatrace.com>

CHANGELOG.md

@@ -10,6 +10,7 @@ the release.
 - Extend semantic conventions for RPC and allow non-gRPC calls ([#604](https://github.com/open-telemetry/opentelemetry-specification/pull/604))
 - Add span attribute to indicate cold starts of Function as a Service executions ([#650](https://github.com/open-telemetry/opentelemetry-specification/pull/650))
 - Added conventions for naming of exporter packages
+- Add semantic conventions for container id
 - Add semantic conventions for HTTP content length
 - Clarify Tracer vs TracerProvider in tracing API and SDK spec. Most importantly:
   * Configuration should be stored not per Tracer but in the TracerProvider.

specification/resource/semantic_conventions/README.md

@@ -93,9 +93,11 @@ Attributes defining a compute unit (e.g. Container, Process, Function as a Servi
 | Attribute  | Description  | Example  |
 |---|---|---|
 | container.name | Container name. | `opentelemetry-autoconf` |
+| container.id | Container id. Usually a UUID, as for example used to [identify Docker containers][]. The UUID might be abbreviated. | `a3bf90e006b2` |
 | container.image.name | Name of the image the container was built on. | `gcr.io/opentelemetry/operator` |
 | container.image.tag | Container image tag. | `0.1` |
 
+[identify Docker containers]: https://docs.docker.com/engine/reference/run/#container-identification
 ### Function as a Service
 
 **type:** `faas`