Fully document external-logs feature

[DutchEllie]

Feature request

In Tekton Dashboard there exists a feature called external-logs which can be used to retrieve logs for a TaskRun from an external source as a fallback should the associated pod be no longer available. The only documentation that exists for this feature is currently in the form of a walkthrough, centered around using some form of object storage and some Banzaicloud logging thingamajig. While it's not too difficult at all to adapt this walkthrough for something like Loki (in fact, I kinda like that practical approach to documentation), the nature of this walkthrough makes it so that documentation is lacking somewhat.

Case in point, the startTime and completionTime feature that was added about a year ago. The PR for this feature neglects to fulfill the documentation part of the submitter checklist with the reason "No existing docs for external-logs aside from the walkthrough. This change doesn't cause a change to those docs.". Now, this makes sense, as neither a startTime nor a completionTime is needed for the way the Banzaicloud operator records the logs to object storage. However, it would have greatly saved me time if the existence of the startTime and completionTime parameters was at the very least mentioned in this walkthrough document.

My request is thus as follows: Please provide a reference in the documentation for all the parameters passed to the external logs provider. That being whatever the getExternalLogURL function does.

Use case

A cluster operator providing Tekton services to their users might want to adapt their currently in-use logging service to fit Tekton Dashboard with the help of the documentation. I imagine most operators have already deployed their own logging service of choice and are simply looking to connect Tekton Dashboard to their own logging infrastructure.

[AlanGreene]

Thanks for the request @DutchEllie. If you're willing to submit a PR to update the documentation I'd be happy to review it.