Add support for earliest aggregatorMergeStrategy (#14598)

* Add EARLIEST aggregator merge strategy.

- More unit tests.
- Include the aggregators analysis type by default in tests.

* Docs.

* Some comments and a test

* Collapse into individual code blocks.

docs/querying/segmentmetadataquery.md

@@ -62,7 +62,7 @@ There are several main parts to a segment metadata query:
 |merge|Merge all individual segment metadata results into a single result|no|
 |context|See [Context](../querying/query-context.md)|no|
 |analysisTypes|A list of Strings specifying what column properties (e.g. cardinality, size) should be calculated and returned in the result. Defaults to ["cardinality", "interval", "minmax"], but can be overridden with using the [segment metadata query config](../configuration/index.md#segmentmetadata-query-config). See section [analysisTypes](#analysistypes) for more details.|no|
-|aggregatorMergeStrategy| The strategy Druid uses to merge aggregators across segments. If true and if the `aggregators` analysis type is enabled, `aggregatorMergeStrategy` defaults to `strict`. Possible values include `strict`, `lenient`, and `latest`. See [`aggregatorMergeStrategy`](#aggregatormergestrategy) for details.|no|
+|aggregatorMergeStrategy| The strategy Druid uses to merge aggregators across segments. If true and if the `aggregators` analysis type is enabled, `aggregatorMergeStrategy` defaults to `strict`. Possible values include `strict`, `lenient`, `earliest`, and `latest`. See [`aggregatorMergeStrategy`](#aggregatormergestrategy) for details.|no|
 |lenientAggregatorMerge|Deprecated. Use `aggregatorMergeStrategy` property instead. If true, and if the `aggregators` analysis type is enabled, Druid merges aggregators leniently.|no|
 
 The format of the result is:
@@ -186,7 +186,7 @@ Currently, there is no API for retrieving this information.
 * `aggregators` in the result will contain the list of aggregators usable for querying metric columns. This may be
 null if the aggregators are unknown or unmergeable (if merging is enabled).
 
-* Merging can be `strict`, `lenient`, or `latest`. See [`aggregatorMergeStrategy`](#aggregatormergestrategy) for details.
+* Merging can be `strict`, `lenient`, `earliest`, or `latest`. See [`aggregatorMergeStrategy`](#aggregatormergestrategy) for details.
 
 * The form of the result is a map of column name to aggregator.
 
@@ -201,10 +201,12 @@ Conflicts between aggregator metadata across segments can occur if some segments
 two segments use incompatible aggregators for the same column, such as `longSum` changed to `doubleSum`.
 Druid supports the following aggregator merge strategies:
 
-- `strict`:  If there are any segments with unknown aggregators or any conflicts of any kind, the merged aggregators
+- `strict`: If there are any segments with unknown aggregators or any conflicts of any kind, the merged aggregators
   list is `null`.
-- `lenient`:  Druid ignores segments with unknown aggregators. Conflicts between aggregators set the aggregator for that particular column to null.
-- the aggregator for that particular column.
+- `lenient`: Druid ignores segments with unknown aggregators. Conflicts between aggregators set the aggregator for
+  that particular column to null.
+- `earliest`: In the event of conflicts between segments, Druid selects the aggregator from the earliest segment
+  for that particular column.
 - `latest`: In the event of conflicts between segments, Druid selects the aggregator from the most recent segment
    for that particular column.
 

processing/src/main/java/org/apache/druid/query/metadata/SegmentMetadataQueryQueryToolChest.java

@@ -277,6 +277,7 @@ public static SegmentAnalysis mergeAnalyses(
 
     SegmentId mergedSegmentId = null;
 
+    // Union datasources can have multiple datasources. So we iterate through all the datasources to parse the segment id.
     for (String dataSource : dataSources) {
       final SegmentId id1 = SegmentId.tryParse(dataSource, arg1.getId());
       final SegmentId id2 = SegmentId.tryParse(dataSource, arg2.getId());
@@ -364,15 +365,23 @@ public static SegmentAnalysis mergeAnalyses(
           aggregators.put(aggregator.getName(), aggregator);
         }
       }
+    } else if (AggregatorMergeStrategy.EARLIEST == aggregatorMergeStrategy) {
+      // The segment analyses are already ordered above, where arg1 is the analysis pertaining to the latest interval
+      // followed by arg2. So for earliest strategy, the iteration order should be arg2 and arg1.
+      for (SegmentAnalysis analysis : ImmutableList.of(arg2, arg1)) {
+        if (analysis.getAggregators() != null) {
+          for (Map.Entry<String, AggregatorFactory> entry : analysis.getAggregators().entrySet()) {
+            aggregators.putIfAbsent(entry.getKey(), entry.getValue());
+          }
+        }
+      }
     } else if (AggregatorMergeStrategy.LATEST == aggregatorMergeStrategy) {
       // The segment analyses are already ordered above, where arg1 is the analysis pertaining to the latest interval
-      // followed by arg2.
+      // followed by arg2. So for latest strategy, the iteration order should be arg1 and arg2.
       for (SegmentAnalysis analysis : ImmutableList.of(arg1, arg2)) {
         if (analysis.getAggregators() != null) {
           for (Map.Entry<String, AggregatorFactory> entry : analysis.getAggregators().entrySet()) {
-            final String aggregatorName = entry.getKey();
-            final AggregatorFactory aggregator = entry.getValue();
-            aggregators.putIfAbsent(aggregatorName, aggregator);
+            aggregators.putIfAbsent(entry.getKey(), entry.getValue());
           }
         }
       }

processing/src/main/java/org/apache/druid/query/metadata/metadata/AggregatorMergeStrategy.java

@@ -28,6 +28,7 @@ public enum AggregatorMergeStrategy
 {
   STRICT,
   LENIENT,
+  EARLIEST,
   LATEST;
 
   @JsonValue