Known Limitation: COS is not available on machine models

It is possible for a machine charm (e.g. `opentelemetry-collector`) to block until it receives certain integrations, which at the moment are only provided by COS charms. COS charms are only available for K8s models, so runs which involve charms like `opentelemetry-collector` and which are scheduled for machine environments cannot succeed.

Example:
https://test-observer.canonical.com/#/charms/136868?testExecutionId=686919&testResultId=11906592

That said, in the example above, we have the option of cross-model relations, so it is worth adding constraint overrides to charms like `opentelemetry-collector`, which should allow them to succeed in cross-model scenarios between machine and Kubernetes environments, while failing fast in machine-only environments.

No file or test is named; start by locating the constraint-override handling and existing tests for machine and Kubernetes model scheduling. Use the opentelemetry-collector example as context. Done means cross-model machine/Kubernetes runs can succeed, while machine-only runs fail fast when COS integrations are unavailable.

Written by the indexing model from the issue text.