interface.otlp: Enable forwarding dashboards in the databag

We need to decide if we want to feature dashboards in the otlp interface. This could be pushing us in a direction of an "observability" i.e., cos-agent-like interface. Relevant thread:
- https://github.com/canonical/charmlibs/pull/352#issuecomment-4033328839

Start by reading the interface.otlp databag context and the relevant discussion in PR 352, especially the linked issue comment. Determine whether forwarding dashboards belongs in this interface and document the chosen direction, including what the databag should represent. Done means the design decision is recorded clearly enough to guide implementation.

Written by the indexing model from the issue text.