[Discover] Fix field stats for epoch time format (#148288)

Closes #148140

## Summary

This PR fixes time format so it can work also for epoch.

Before:
<img width="400" alt="Screenshot 2023-01-03 at 12 33 17"
src="https://user-images.githubusercontent.com/1415710/210350243-b3ae1d1a-4c76-46e7-930f-0ca9835dc121.png">

After:
<img width="400" alt="Screenshot 2023-01-03 at 12 32 49"
src="https://user-images.githubusercontent.com/1415710/210350274-af80f558-6a32-4da6-9cb9-857a8cb3de70.png">

Sample data for testing:
```
PUT test
PUT test/_mapping
{
    "properties": {
        "timestamp": {
            "type": "date",
            "format": "epoch_second"
        },
        "message":  {
          "type": "keyword"
        },
        "bytes": {
          "type": "long"
        }
    }
}
    
POST test/_doc/1
{
    "timestamp": 1669912088.9230318,
    "message": "Message 1",
    "bytes": 250
}
POST test/_doc/2
{
    "timestamp": 1669912088.9230319,
    "message": "Message 2",
    "bytes": 20
}
```

src/plugins/unified_field_list/common/utils/field_stats_utils.test.ts

@@ -0,0 +1,153 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import { buildSearchParams } from './field_stats_utils';
+
+describe('fieldStatsUtils', function () {
+  describe('buildSearchParams()', () => {
+    it('should work correctly for aggregations and a data view time field', () => {
+      expect(
+        buildSearchParams({
+          dataViewPattern: 'kibana_sample_data_logs',
+          timeFieldName: 'timestamp',
+          fromDate: '2022-12-05T23:00:00.000Z',
+          toDate: '2023-01-05T09:33:05.359Z',
+          dslQuery: {
+            bool: {
+              must: [],
+              filter: [
+                {
+                  match_phrase: {
+                    'geo.src': 'US',
+                  },
+                },
+              ],
+              should: [],
+              must_not: [],
+            },
+          },
+          runtimeMappings: {
+            hour_of_day: {
+              type: 'long',
+              script: {
+                source: "emit(doc['timestamp'].value.getHour());",
+              },
+            },
+          },
+          aggs: {
+            sample: {
+              sampler: {
+                shard_size: 5000,
+              },
+              aggs: {
+                sample_count: {
+                  value_count: {
+                    field: 'extension.keyword',
+                  },
+                },
+                top_values: {
+                  terms: {
+                    field: 'extension.keyword',
+                    size: 10,
+                    shard_size: 25,
+                  },
+                },
+              },
+            },
+          },
+        })
+      ).toMatchSnapshot();
+    });
+
+    it('should work correctly for aggregations without a data view time field', () => {
+      expect(
+        buildSearchParams({
+          dataViewPattern: 'kibana_sample*',
+          timeFieldName: '',
+          fromDate: '2022-12-05T23:00:00.000Z',
+          toDate: '2023-01-05T09:33:53.717Z',
+          dslQuery: {
+            bool: {
+              must: [],
+              filter: [
+                {
+                  match_phrase: {
+                    'geo.src': 'US',
+                  },
+                },
+              ],
+              should: [],
+              must_not: [],
+            },
+          },
+          runtimeMappings: {},
+          aggs: {
+            sample: {
+              sampler: {
+                shard_size: 5000,
+              },
+              aggs: {
+                sample_count: {
+                  value_count: {
+                    field: 'extension.keyword',
+                  },
+                },
+                top_values: {
+                  terms: {
+                    field: 'extension.keyword',
+                    size: 10,
+                    shard_size: 25,
+                  },
+                },
+              },
+            },
+          },
+        })
+      ).toMatchSnapshot();
+    });
+
+    it('should work correctly for fetching field examples', () => {
+      expect(
+        buildSearchParams({
+          dataViewPattern: 'kibana_sample_data_logs',
+          timeFieldName: 'timestamp',
+          fromDate: '2022-12-05T23:00:00.000Z',
+          toDate: '2023-01-05T09:35:24.109Z',
+          dslQuery: {
+            bool: {
+              must: [],
+              filter: [
+                {
+                  match_phrase: {
+                    'geo.src': 'US',
+                  },
+                },
+              ],
+              should: [],
+              must_not: [],
+            },
+          },
+          runtimeMappings: {
+            hour_of_day: {
+              type: 'long',
+              script: {
+                source: "emit(doc['timestamp'].value.getHour());",
+              },
+            },
+          },
+          fields: [
+            {
+              field: '_id',
+            },
+          ],
+          size: 100,
+        })
+      ).toMatchSnapshot();
+    });
+  });
+});

src/plugins/unified_field_list/common/utils/field_stats_utils.ts

@@ -55,6 +55,7 @@ export function buildSearchParams({
             [timeFieldName]: {
               gte: fromDate,
               lte: toDate,
+              format: 'strict_date_optional_time',
             },
           },
         },