ObservedTimestamp should be set when using logs SDK

[owent]

Describe your environment

otel-cpp: 1.8.2

What is the expected behavior?
ObservedTimestamp should be set when using logs SDK

What is the actual behavior?
ObservedTimestamp not set

Additional context

From: https://cloud-native.slack.com/archives/C01N3AT62SJ/p1676338373857439

[ArielGMachado]

@owent yesterday I couldn't file this issue because of late hour.
Evidence of this issue about ObservedTimestamp not set by building the examples/otlp/example_otlp_grpc_log and running an otel-col to show the logs:

otel-collector           | 2023-02-15T09:53:09.201Z	info	LogsExporter	{"kind": "exporter", "data_type": "logs", "name": "logging", "#logs": 1}
otel-collector           | 2023-02-15T09:53:09.201Z	info	ResourceLog #0
otel-collector           | Resource SchemaURL: 
otel-collector           | Resource attributes:
otel-collector           |      -> service.name: Str(unknown_service)
otel-collector           |      -> telemetry.sdk.version: Str(1.8.2)
otel-collector           |      -> telemetry.sdk.name: Str(opentelemetry)
otel-collector           |      -> telemetry.sdk.language: Str(cpp)
otel-collector           | ScopeLogs #0
otel-collector           | ScopeLogs SchemaURL: 
otel-collector           | InstrumentationScope foo_library 
otel-collector           | LogRecord #0
otel-collector           | ObservedTimestamp: 1970-01-01 00:00:00 +0000 UTC
otel-collector           | Timestamp: 2023-02-15 09:53:02.639227236 +0000 UTC
otel-collector           | SeverityText: DEBUG
otel-collector           | SeverityNumber: Debug(5)
otel-collector           | Body: Str(body)
otel-collector           | Trace ID: 8af31c6478c798672bdfa524b72e392b
otel-collector           | Span ID: c1751d20ac6b8f16
otel-collector           | Flags: 1
otel-collector           | 	{"kind": "exporter", "data_type": "logs", "name": "logging"}