otelcol: make Factories.Telemetry required

[axw]

Component(s)

otelcol

Describe the issue you're reporting

See #13950 (comment)

To keep the code maintainable, we should make a breaking change and require callers of otelcol to provide a telemetry factory. We'll initially provide a default (otelconftelemetry) if none is provided, to ease the transition.

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[axw]

@mx-psi suggested in #13950 (comment):

I think we can even hint the user to use otelconftelemetry.NewFactory, since users seeing this error message most likely would want to use that factory.