Add ObservedTimestamp to the Log Data Model

Resolves #1875

See the issue for the discussion and the description of the source vs observed timestamps.

CHANGELOG.md

@@ -26,6 +26,9 @@ release.
 
 ### Logs
 
+- Added ObservedTimestamp to the Log Data Model.
+  ([#2184](https://github.com/open-telemetry/opentelemetry-specification/pull/2184))
+
 ### Resource
 
 ### Semantic Conventions

specification/logs/data-model.md

@@ -178,6 +178,7 @@ Here is the list of fields in a log record:
 Field Name     |Description
 ---------------|--------------------------------------------
 Timestamp      |Time when the event occurred.
+ObservedTimestamp|Time when the event was observed.
 TraceId        |Request trace id.
 SpanId         |Request span id.
 TraceFlags     |W3C trace flag.
@@ -197,6 +198,24 @@ Type: Timestamp, uint64 nanoseconds since Unix epoch.
 Description: Time when the event occurred measured by the origin clock. This
 field is optional, it may be missing if the timestamp is unknown.
 
+### Field: `ObservedTimestamp`
+
+Type: Timestamp, uint64 nanoseconds since Unix epoch.
+
+Description: Time when the event was observed by the collection system. For
+events that originate in OpenTelemetry this timestamp is typically set at the
+generation time and is equal to Timestamp. For events originating externally and
+collected by OpenTelemetry (e.g. using Collector) this is the time when the
+OpenTelemetry's code observed the event measure by the clock of the
+OpenTelemetry code. This field SHOULD be set once the event is observed by
+OpenTelemetry.
+
+For converting OpenTelemetry log data to formats that support only one timestamp
+or when receiving OpenTelemetry log data by recipients that support only one
+timestamp internally the following logic is recommended:
+
+- Use `Timestamp` if it is present, otherwise use `ObservedTimestamp`.
+
 ### Trace Context Fields
 
 #### Field: `TraceId`