fix typo in OpenMetrics type string for gaugehistogram

Author: fstab

simpleclient_common/src/main/java/io/prometheus/client/exporter/common/TextFormat.java

@@ -291,7 +291,7 @@ private static String omTypeString(Collector.Type t) {
       case HISTOGRAM:
         return "histogram";
       case GAUGE_HISTOGRAM:
-        return "gauge_histogram";
+        return "gaugehistogram";
       case STATE_SET:
         return "stateset";
       case INFO: