logback-android/src/main/java/ch/qos/logback/classic/pattern/MDCConverter.java
private String outputMDCForAllKeys(Map<String, String> mdcPropertyMap) {
    StringBuilder buf = new StringBuilder();
    boolean first = true;
    for (Map.Entry<String, String> entry : mdcPropertyMap.entrySet()) {
      if (first) {
        first = false;
      } else {
        buf.append(", ");
      }
      //format: key0=value0, key1=value1
      buf.append(entry.getKey()).append('=').append(entry.getValue());
    }
    return buf.toString();
  }
private String outputMDCForAllKeys(Map<String, String> mdcPropertyMap) {
    StringBuilder buf = new StringBuilder();
    boolean first = true;
    for (Map.Entry<String, String> entry : mdcPropertyMap.entrySet()) {
      if (first) {
        first = false;
      } else {
        buf.append(", ");
      }
      //format: key0=value0, key1=value1
      buf.append(entry.getKey()).append('=').append(entry.getValue());
    }
    return buf.toString();
  }
if no key is specified, return all the values present in the MDC, in the format "k1=v1, k2=v2, ..."