Logs: ObservedTimestamp field is not set

[tigrannajaryan]

Describe your environment
Mac OS X
Python 3.9.6

Steps to reproduce
Run Getting Started, metrics example.

What is the expected behavior?
ObservedTimestamp value set equal to Timestamp as required by the spec.

What is the actual behavior?
ObservedTimestamp value is set to 1970-01-01 00:00:00 +0000 UTC.

Collector output:

LogRecord #1
ObservedTimestamp: 1970-01-01 00:00:00 +0000 UTC
Timestamp: 2023-11-22 17:26:29.306744064 +0000 UTC
SeverityText: INFO
SeverityNumber: Info(9)
Body: Str(Press CTRL+C to quit)
Attributes:
     -> otelSpanID: Str(0)
     -> otelTraceID: Str(0)
     -> otelTraceSampled: Bool(false)
     -> otelServiceName: Str(dice-server)
Trace ID:
Span ID:
Flags: 0

Additional context
Part of review open-telemetry/community#1751