Better messaging around missing env var

Currently, the messaging when you are missing the OTEL_FUNCTIONS_WORKER_ENABLED is not specific to this env var and leads customers to believe they are missing environment vars defined by the OTEL spec. Change this to call out the name of this env var specifically

No file or test is named. Start by locating the code path that reports a missing OTEL_FUNCTIONS_WORKER_ENABLED variable, then update the message so it names that variable specifically and verify the missing-variable output no longer suggests unspecified OpenTelemetry environment variables.

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