Initial POC for metric aggregations

src/main/kotlin/mbuhot/eskotlin/aggregation/metrics/AverageAggregation.kt

@@ -0,0 +1,25 @@
+package mbuhot.eskotlin.aggregation.metrics
+
+import org.elasticsearch.search.aggregations.AggregationBuilder
+import org.elasticsearch.search.aggregations.AggregationBuilders
+import org.elasticsearch.search.aggregations.metrics.avg.AvgAggregationBuilder
+
+data class AverageAggregationData(
+        var name: String? = null,
+        var field: String? = null,
+        var missing: Int? = null
+)
+
+fun averageAggregation(init: AverageAggregationData.() -> Unit): AggregationBuilder {
+    val params = AverageAggregationData().apply(init)
+
+    val agg = AggregationBuilders
+            .avg(params.name)
+            .field(params.field)
+
+    return if (params.missing != null) {
+        agg.missing(params.missing)
+    } else {
+        agg
+    }
+}

src/test/kotlin/mbuhot/eskotlin/aggregation/metrics/AverageAggregationTest.kt

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+package mbuhot.eskotlin.aggregation.metrics
+
+import mbuhot.eskotlin.query.should_render_as
+import org.junit.Test
+
+class AverageAggregationTest {
+    @Test
+    fun `test average aggregation`() {
+        val agg = averageAggregation {
+            name = "avg_grade"
+            field = "grade"
+        }
+
+        agg should_render_as """
+            {
+                "avg_grade" : { "avg" : { "field" : "grade" } }
+            }
+        """.trimIndent()
+    }
+
+    @Test
+    fun `test average aggregation with missing value`() {
+        val agg = averageAggregation {
+            name = "avg_grade"
+            field = "grade"
+            missing = 10
+        }
+
+        agg should_render_as """
+            {
+                "avg_grade" : { "avg": { "field" : "grade", "missing" : 10 } }
+            }
+        """.trimIndent()
+    }
+}