[exporter/<logs>] Update timestamp processing logic

[djaglowski]

The log data model was recently update to include a second timestamp. In order for logging exporters to maintain their current functionality, they must be updated to use Timestamp when it is present, and ObservedTimestamp otherwise.

A quick search appears to show that the following exporters may be affected:

• azuremonitorexporter
• observiqexporter
• lokiexporter

This issue is dependent upon collector#5171 being completed first.

[jpkrohling]

cc @mar4uk

[mar4uk]

@djaglowski Do you plan to work on this issue by yourself? Or I can assign it to myself and do it?

[djaglowski]

@mar4uk, your help would be very appreciated, thank you! ! I'll assign it to you.