enable opentelemetry optionally

I need to revert https://github.com/elastic/ai-github-actions/pull/1617 since it will enforce the usage of the opentelemetry and those secrets.

But that won't be possible in some of the consumers.

Therefore, I want the observability fragment to be configured optionally, explore if that possible and how to do it

Start by reviewing pull request 1617 and the observability fragment it introduced, then trace how consumers are required to provide OpenTelemetry and its secrets. Define how the fragment can be configured optionally and verify that consumers without those secrets remain supported. Done means the approach is documented clearly enough to implement and the enforced dependency can be reverted.

Written by the indexing model from the issue text.