Ensure optional dependencies are optional

[pquentin]

orjson is an optional dependency, but I was checking its absence with ModuleNotFoundError, a child exception of ImportError. While import orjson will indeed raise ModuleNotFoundError, from .serializer import OrjsonSerializer raises ImportError instead because the module (.serializer) was found.

OpenTelemetry is already tested in separate integration tests, but checking it explicitly in lint seemed useful as it's only one more line.

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[ezimuel]

LGTM!