Add span attribute to indicate cold starts of Function as a Service e…

…xecutions (open-telemetry#650)

* Add `faas.coldstart` attribute

* Update changelog

CHANGELOG.md

@@ -7,6 +7,7 @@ the release.
 
 ## Unreleased
 
+- 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
 - Clarify Tracer vs TracerProvider in tracing API and SDK spec. Most importantly:
   * Configuration should be stored not per Tracer but in the TracerProvider.

specification/trace/semantic_conventions/faas.md

@@ -27,6 +27,7 @@ If Spans following this convention are produced, a Resource of type `faas` MUST
 |---|---|--|
 | `faas.trigger` | Type of the trigger on which the function is executed. <br > It SHOULD be one of the following strings: "datasource", "http", "pubsub", "timer", or "other". | Yes |
 | `faas.execution` | String containing the execution id of the function. E.g. `af9d5aa4-a685-4c5f-a22b-444f80b3cc28` | No |
+| `faas.coldstart` | A boolean indicating that the serverless function is executed for the first time (aka cold start). | No |
 
 ### Difference between execution and instance