Handle DateTimeOffset in the same way as DateTime

[reyang]

Many .NET users are adopting DateTimeOffset, there seem to be a common expectation that DateTimeOffset should be handled in the same way as DateTime.

Additional context:

• https://stackoverflow.com/questions/4331189/datetime-vs-datetimeoffset
• https://github.com/open-telemetry/opentelemetry-dotnet/blob/943c856c1c6295a536bc8a1c89896febc0be6107/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/TimestampHelpers.cs