OTLP exporter: Set observed timestamp

[alanwest]

Observed timestamp should equal the timestamp. See: https://github.com/open-telemetry/opentelemetry-proto/blob/395c8422fe90080314c7d9b4114d701a0c049e1f/opentelemetry/proto/logs/v1/logs.proto#L123-L129

  // time_unix_nano is the time when the event occurred.
  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January 1970.
  // Value of 0 indicates unknown or missing timestamp.
  fixed64 time_unix_nano = 1;

  // Time when the event was observed by the collection system.
  // For events that originate in OpenTelemetry (e.g. using OpenTelemetry Logging SDK)
  // 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 OpenTelemetry's code observed the event measured
  // by the clock of the OpenTelemetry code. This field MUST 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 time_unix_nano if it is present, otherwise use observed_time_unix_nano.
  //
  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January 1970.
  // Value of 0 indicates unknown or missing timestamp.
  fixed64 observed_time_unix_nano = 11;

[codecov]

Codecov Report

Merging #4444 (449cd71) into main (9d51cc3) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4444      +/-   ##
==========================================
- Coverage   85.00%   84.98%   -0.03%     
==========================================
  Files         306      306              
  Lines       12182    12184       +2     
==========================================
- Hits        10355    10354       -1     
- Misses       1827     1830       +3     

Impacted Files	Coverage Δ
...etryProtocol/Implementation/LogRecordExtensions.cs	89.41% <100.00%> (+0.25%)	⬆️

... and 6 files with indirect coverage changes

[ArielGMachado]

As context, this issue was discussed here: https://cloud-native.slack.com/archives/C01N3BC2W7Q/p1682531179925309