sql: add created_at to index usage stat telemetry

This commit adds a creation timestamp to the index usage
statistics telemetry.

Fixes cockroachdb#84458.

Release justification: low-risk updates to new functionality
Release note(sql change): Added a creation timestamp to index usage statistics
telemetry.

docs/generated/eventlog.md

@@ -2469,6 +2469,7 @@ An event of type `captured_index_usage_stats`
 | `IndexType` |  | no |
 | `IsUnique` |  | no |
 | `IsInverted` |  | no |
+| `CreatedAt` |  | no |
 
 
 #### Common fields

pkg/sql/scheduledlogging/captured_index_usage_stats.go

@@ -171,7 +171,7 @@ func captureIndexUsageStats(
 
 	// Capture index usage statistics for each database.
 	var ok bool
-	expectedNumDatums := 9
+	expectedNumDatums := 10
 	var allCapturedIndexUsageStats []logpb.EventPayload
 	for _, databaseName := range allDatabaseNames {
 		// Omit index usage statistics on the default databases 'system',
@@ -189,7 +189,8 @@ func captureIndexUsageStats(
 		 ti.is_unique,
 		 ti.is_inverted,
 		 total_reads,
-		 last_read
+		 last_read,
+		 ti.created_at
 		FROM %s.crdb_internal.index_usage_statistics AS us
 		JOIN %s.crdb_internal.table_indexes ti
 		ON us.index_id = ti.index_id
@@ -233,6 +234,10 @@ func captureIndexUsageStats(
 			if row[8] != tree.DNull {
 				lastRead = tree.MustBeDTimestampTZ(row[8]).Time
 			}
+			createdAt := time.Time{}
+			if row[9] != tree.DNull {
+				createdAt = tree.MustBeDTimestamp(row[9]).Time
+			}
 
 			capturedIndexStats := &eventpb.CapturedIndexUsageStats{
 				TableID:        uint32(roachpb.TableID(tableID)),
@@ -245,6 +250,7 @@ func captureIndexUsageStats(
 				IndexType:      string(indexType),
 				IsUnique:       bool(isUnique),
 				IsInverted:     bool(isInverted),
+				CreatedAt:      createdAt.String(),
 			}
 
 			allCapturedIndexUsageStats = append(allCapturedIndexUsageStats, capturedIndexStats)

pkg/util/log/eventpb/telemetry.proto

@@ -171,6 +171,7 @@ message CapturedIndexUsageStats {
   string index_type = 9 [(gogoproto.jsontag) = ",omitempty", (gogoproto.moretags) = "redact:\"nonsensitive\""];
   bool is_unique = 10 [(gogoproto.jsontag) = ",omitempty"];
   bool is_inverted = 11 [(gogoproto.jsontag) = ",omitempty"];
+  string created_at = 12 [(gogoproto.jsontag) = ",omitempty", (gogoproto.moretags) = "redact:\"nonsensitive\""];
 }
 
 // CreateChangefeed is an event for any CREATE CHANGEFEED query that