refactor ConfigurationLoader

@Getter

public final class ConfigurationLoader {

    private static final ConfigurationParser[] PARSERS = new ConfigurationParser[]{new OpentracingConfigurationParser()};

    private final String tracerClassName;

    @Getter

    private final int sampleNumPM;

    public ConfigurationLoader() {

        String tracerClassName = null;

        int maxSampleNum = 0;

        for (ConfigurationParser each : PARSERS) {

            Optional<String> sampleNumPMOptional = each.parse("tracer.sampleNumPM");

            if (sampleNumPMOptional.isPresent()) {

                maxSampleNum = Integer.parseInt(sampleNumPMOptional.get());

            }

            Optional<String> tracerClassOptional = each.parse("tracer.class");

            if (tracerClassOptional.isPresent()) {

                tracerClassName = tracerClassOptional.get();

                break;

            }

        }

        Preconditions.checkNotNull(tracerClassName);

        this.tracerClassName = tracerClassName;

        this.sampleNumPM = maxSampleNum;

        ConfigurationParser configurationParser = new OpentracingConfigurationParser();

        Optional<String> tracerClass = configurationParser.parse("tracer.class");

        Preconditions.checkState(tracerClass.isPresent(), "Can not find opentracing tracer implementation class.");

        this.tracerClassName = tracerClass.get();

    }

}