add aggregator for parallel requests

ParallelRequestsAggregator generates a line plot that shows the number
of parallel requests among the plotted events.
This plot has two issues:
1. It only considers events that are plotted. Events that occur later,
   but were started within the plotted time frame are not considered.
2. For performance reasons we are only plotting points when a value
   changed. This leads to diagonal lines.

pdb-ui/src/main/java/org/lucares/pdbui/PlotSettingsTransformer.java

@@ -2,6 +2,7 @@ package org.lucares.pdbui;
 
 import org.lucares.pdb.plot.api.AggregateHandler;
 import org.lucares.pdb.plot.api.NullAggregate;
+import org.lucares.pdb.plot.api.ParallelRequestsAggregate;
 import org.lucares.pdb.plot.api.PercentileAggregate;
 import org.lucares.pdb.plot.api.PlotSettings;
 import org.lucares.pdb.plot.api.TimeRangeUnitInternal;
@@ -59,6 +60,8 @@ class PlotSettingsTransformer {
 			return new NullAggregate();
 		case PERCENTILES:
 			return new PercentileAggregate();
+		case PARALLEL:
+			return new ParallelRequestsAggregate();
 		}
 		throw new IllegalStateException("unhandled enum: " + aggregate);
 	}

pdb-ui/src/main/java/org/lucares/pdbui/domain/Aggregate.java

@@ -1,5 +1,5 @@
 package org.lucares.pdbui.domain;
 
 public enum Aggregate {
-	NONE, PERCENTILES
+	NONE, PERCENTILES, PARALLEL
 }

pdb-ui/src/main/resources/log4j2.xml

@@ -43,6 +43,7 @@
   	<logger name="org.lucares.metrics.proposals" level="DEBUG" />
   	<logger name="org.lucares.metrics.plotter" level="DEBUG" />
   	<logger name="org.lucares.metrics.gnuplot" level="DEBUG" />
+  	<logger name="org.lucares.metrics.aggregator.parallelRequests" level="DEBUG" />
   	<!-- 
   	<logger name="org.lucares.metrics.ingestion.tagsToFile.newPdbWriter" level="DEBUG" />
   	<logger name="org.lucares.metrics.dataStore" level="DEBUG" />

pdb-ui/src/main/resources/resources/js/ui.js

@@ -783,6 +783,7 @@ Vue.component('search-bar', {
 				<select id="show-aggregate" v-model="searchBar.aggregate">
 					<option value="NONE">-</option>
 					<option value="PERCENTILES">percentiles</option>
+					<option value="PARALLEL">parallel requests</option>
 				</select>
 			</div>