datadog-agent [trace] Ensure dogstatsd is enabled before emitting trace metrics

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Fixes https://github.com/DataDog/datadog-agent/issues/8041

Add a check for whether dogstatsd is enabled before emitting any trace metrics. This ensures that dogstatsd-formatted metrics are not accidentally leaked to another statsd listener in the event that the default DD_DOGSTATSD_PORT value is set but DD_USE_DOGSTATSD is disabled. Fixes #8041

This may be an unexpected behavior change in some corner configuration cases, but I believe that by the phrasing of DD_USE_DOGSTATSD this is the correct behavior if that setting is false.
- Also: this is consistent with another check within the trace package: https://github.com/DataDog/datadog-agent/blob/main/pkg/trace/api/dogstatsd.go#L22-L27

Added a new unit test to the suite for this case, and the existing ones should continue to pass.

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