[8.15](backport #41640) libbeat: increase total_fields.limit to 12500 (#41672)

It increased the `index.mapping.total_fields.limit` from `10000` to `12500` in order to avoid ingestion failures caused by too many field in the index. Since 8.15.0 the limit started to be hit.
The field count being exceeded is on the index, counting all mapped fields and the dynamic fields. That's why a small event might trigger the error, the event contains new fields to be mapped which would exceed the total field limit if mapped.

(cherry picked from commit 42dd93b)
---------

Co-authored-by: Anderson Queiroz <[email protected]>

Author: mergify[bot]

CHANGELOG.next.asciidoc

@@ -10,6 +10,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 
 *Affecting all Beats*
 
+- Fix metrics not being ingested, due to "Limit of total fields [10000] has been exceeded while adding new fields [...]". The total fields limit has been increased to 12500. No significant performance impact on Elasticsearch is anticipated. {pull}41640[41640]
 
 *Auditbeat*
 

libbeat/template/load_test.go

@@ -170,7 +170,7 @@ func TestFileLoader_Load(t *testing.T) {
 							"refresh_interval": "5s",
 							"mapping": mapstr.M{
 								"total_fields": mapstr.M{
-									"limit": 10000,
+									"limit": defaultTotalFieldsLimit,
 								},
 							},
 							"query": mapstr.M{

libbeat/template/template.go

@@ -35,7 +35,7 @@ import (
 var (
 	// Defaults used in the template
 	defaultDateDetection           = false
-	defaultTotalFieldsLimit        = 10000
+	defaultTotalFieldsLimit        = 12500
 	defaultMaxDocvalueFieldsSearch = 200
 
 	defaultFields []string