otelcol: make Factories.Telemetry required

[axw]

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Component(s)

otelcol

Describe the issue you're reporting

See https://github.com/open-telemetry/opentelemetry-collector/pull/13950#discussion_r2425393378

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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