Support offset in composite aggs (elastic#50609)

Adds support for the `offset` parameter to the `date_histogram` source
of composite aggs. The `offset` parameter is supported by the normal
`date_histogram` aggregation and is useful for folks that need to
measure things from, say, 6am one day to 6am the next day.

This is implemented by creating a new `Rounding` that knows how to
handle offsets and delegates to other rounding implementations. That
implementation doesn't fully implement the `Rounding` contract, namely
`nextRoundingValue`. That method isn't used by composite aggs so I can't
be sure that any implementation that I add will be correct. I propose to
leave it throwing `UnsupportedOperationException` until I need it.

Closes elastic#48757

docs/reference/aggregations/bucket/composite-aggregation.asciidoc

@@ -287,6 +287,72 @@ Time zones may either be specified as an ISO 8601 UTC offset (e.g. `+01:00` or
 `-08:00`)  or as a timezone id, an identifier used in the TZ database like
 `America/Los_Angeles`.
 
+*Offset*
+
+include::datehistogram-aggregation.asciidoc[tag=offset-explanation]
+
+[source,console,id=composite-aggregation-datehistogram-offset-example]
+----
+PUT my_index/_doc/1?refresh
+{
+  "date": "2015-10-01T05:30:00Z"
+}
+
+PUT my_index/_doc/2?refresh
+{
+  "date": "2015-10-01T06:30:00Z"
+}
+
+GET my_index/_search?size=0
+{
+  "aggs": {
+    "my_buckets": {
+      "composite" : {
+        "sources" : [
+          {
+            "date": {
+              "date_histogram" : {
+                "field": "date",
+                "calendar_interval": "day",
+                "offset": "+6h",
+                "format": "iso8601"
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}
+----
+
+include::datehistogram-aggregation.asciidoc[tag=offset-result-intro]
+
+[source,console-result]
+----
+{
+  ...
+  "aggregations": {
+    "my_buckets": {
+      "after_key": { "date": "2015-10-01T06:00:00.000Z" },
+      "buckets": [
+        {
+          "key": { "date": "2015-09-30T06:00:00.000Z" },
+          "doc_count": 1
+        },
+        {
+          "key": { "date": "2015-10-01T06:00:00.000Z" },
+          "doc_count": 1
+        }
+      ]
+    }
+  }
+}
+----
+// TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]
+
+include::datehistogram-aggregation.asciidoc[tag=offset-note]
+
 ===== Mixing different values source
 
 The `sources` parameter accepts an array of values source.

docs/reference/aggregations/bucket/datehistogram-aggregation.asciidoc

@@ -461,16 +461,19 @@ the bucket covering that day will only hold data for 23 hours instead of the usu
 where you'll have only a 11h bucket on the morning of 27 March when the DST shift
 happens.
 
+[[search-aggregations-bucket-datehistogram-offset]]
 ===== Offset
 
+// tag::offset-explanation[]
 Use the `offset` parameter to change the start value of each bucket by the
 specified positive (`+`) or negative offset (`-`) duration, such as `1h` for
 an hour, or `1d` for a day. See <<time-units>> for more possible time
 duration options.
 
 For example, when using an interval of `day`, each bucket runs from midnight
-to midnight.  Setting the `offset` parameter to `+6h` changes each bucket
+to midnight. Setting the `offset` parameter to `+6h` changes each bucket
 to run from 6am to 6am:
+// end::offset-explanation[]
 
 [source,console]
 -----------------------------
@@ -498,8 +501,10 @@ GET my_index/_search?size=0
 }
 -----------------------------
 
+// tag::offset-result-intro[]
 Instead of a single bucket starting at midnight, the above request groups the
 documents into buckets starting at 6am:
+// end::offset-result-intro[]
 
 [source,console-result]
 -----------------------------
@@ -525,8 +530,10 @@ documents into buckets starting at 6am:
 -----------------------------
 // TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]
 
+// tag::offset-note[]
 NOTE: The start `offset` of each bucket is calculated after `time_zone`
 adjustments have been made.
+// end::offset-note[]
 
 ===== Keyed Response
 

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/230_composite.yml

@@ -390,6 +390,40 @@ setup:
   - match: { aggregations.test.buckets.0.key.date: "2017-10-21" }
   - match: { aggregations.test.buckets.0.doc_count: 1 }
 
+---
+"Composite aggregation with date_histogram offset":
+  - skip:
+      version: " - 7.5.99"
+      reason:  offset introduced in 7.6.0
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        index: test
+        body:
+          aggregations:
+            test:
+              composite:
+                sources: [
+                  {
+                    "date": {
+                      "date_histogram": {
+                        "field": "date",
+                        "calendar_interval": "1d",
+                        "offset": "4h",
+                        "format": "iso8601" # Format makes the comparisons a little more obvious
+                      }
+                    }
+                  }
+                ]
+
+  - match: {hits.total: 6}
+  - length: { aggregations.test.buckets: 2 }
+  - match: { aggregations.test.buckets.0.key.date: "2017-10-19T04:00:00.000Z" }
+  - match: { aggregations.test.buckets.0.doc_count: 1 }
+  - match: { aggregations.test.buckets.1.key.date: "2017-10-21T04:00:00.000Z" }
+  - match: { aggregations.test.buckets.1.doc_count: 1 }
+
 ---
 "Composite aggregation with after_key in the response":
   - skip:
@@ -702,7 +736,6 @@ setup:
       reason: geotile_grid is not supported until 7.5.0
   - do:
       search:
-        rest_total_hits_as_int: true
         index: test
         body:
           aggregations:
@@ -725,7 +758,8 @@ setup:
                 ]
                 after: { "geo": "12/730/1590", "kw": "foo" }
 
-  - match: {hits.total: 6}
+  - match: { hits.total.value: 6 }
+  - match: { hits.total.relation: "eq" }
   - length: { aggregations.test.buckets: 3 }
   - match: { aggregations.test.buckets.0.key.geo: "12/1236/1533" }
   - match: { aggregations.test.buckets.0.key.kw: "bar" }