Fix usage string of logger level flag (#531) (#533)
Fix usage string of logger level flag (#531) (#533)

Remove invalid TRACE level & add missing DPANIC, PANIC levels to logger
level flag's usage string.
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib