Add MapAggregator from 1 (key, aggregator) pair

- Make API easier to use (more consistent), by allowing to create a MapAggregator of one "metric"

algebird-test/src/test/scala/com/twitter/algebird/TupleAggregatorsTest.scala

@@ -340,6 +340,13 @@ class TupleAggregatorsTest extends WordSpec with Matchers {
 
     val MinLongAgg = Aggregator.min[Int].andThenPresent{ _.toLong }
 
+    "Create an aggregator from 1 (key, aggregator) pair" in {
+      val agg: Aggregator[Int, Long, Map[String, Long]] = MapAggregator(
+        ("key1", SizeAgg))
+      val expectedMap = Map("key1" -> 6)
+      assert(agg(data) == expectedMap)
+    }
+
     "Create an aggregator from 2 (key, aggregator) pairs" in {
       val agg: Aggregator[Int, Tuple2[Int, Long], Map[String, Long]] = MapAggregator(
         ("key1", MinLongAgg),

project/GenTupleAggregators.scala

@@ -66,8 +66,17 @@ object MapAggregator {
   }
 
   def genMapMethods(max: Int, isMonoid: Boolean = false): String = {
+    val aggType = if (isMonoid) "Monoid" else ""
+
+    // there's no Semigroup[Tuple1[T]], so just use T as intermediary type instead of Tuple1[T]
+    val aggregatorForOneItem = s"""
+       |def apply[K, A, B, C](aggDef: (K, ${aggType}Aggregator[A, B, C])): ${aggType}Aggregator[A, B, Map[K, C]] = {
+       |  val (key, agg) = aggDef
+       |  agg.andThenPresent(value => Map(key -> value))
+       |}
+    """.stripMargin
+
     (2 to max).map(i => {
-      val aggType = if (isMonoid) "Monoid" else ""
       val nums = (1 to i)
       val bs = nums.map("B" + _).mkString(", ")
       val aggs = nums.map(x => "agg%s: Tuple2[K, %sAggregator[A, B%s, C]]".format(x, aggType, x)).mkString(", ")
@@ -90,6 +99,6 @@ def apply[K, A, %s, C](%s): %sAggregator[A, %s, Map[K, C]] = {
             prepares,
             semiType, semigroup,
             tupleBs, present)
-    }).mkString("\n")
+    }).mkString("\n") + aggregatorForOneItem
   }
 }