code: protected Double getSampleRate(Dsn dsn) {
          return Util.parseDouble(Lookup.lookup(SAMPLE_RATE_OPTION, dsn), null);
      }
docs: Whether to sample events, and if so how much to allow through to the server (from 0.0 to 1.0).
  
  @param dsn Sentry server DSN which may contain options.
  @return The ratio of events to allow through to server, or null if sampling is disabled.