app/vlinsert/elasticsearch: add fake response for logstash requests (#6742)

### Describe Your Changes

This is needed in order to support standard Elasticsearch output in
Logstash pipelines.

See: https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6660

### Checklist

The following checks are **mandatory**:

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>

app/vlinsert/elasticsearch/elasticsearch.go

@@ -57,6 +57,12 @@ func RequestHandler(path string, w http.ResponseWriter, r *http.Request) bool {
 		fmt.Fprintf(w, `{}`)
 		return true
 	}
+	if strings.HasPrefix(path, "/logstash") || strings.HasPrefix(path, "/_logstash") {
+		// Return fake response for Logstash APIs requests.
+		// See: https://www.elastic.co/guide/en/elasticsearch/reference/8.8/logstash-apis.html
+		fmt.Fprintf(w, `{}`)
+		return true
+	}
 	switch path {
 	case "/":
 		switch r.Method {

docs/VictoriaLogs/CHANGELOG.md

@@ -22,6 +22,8 @@ according to [these docs](https://docs.victoriametrics.com/victorialogs/quicksta
 * FEATURE: [web UI](https://docs.victoriametrics.com/victorialogs/querying/#web-ui): display the number of entries within each log group.
 * FEATURE: [web UI](https://docs.victoriametrics.com/victorialogs/querying/#web-ui): move the Markdown toggle to the general settings panel in the upper left corner.
 
+* BUGFIX: properly handle Logstash requests for Elasticsearch configuration when using `outputs.elasticsearch` in Logstash pipelines. Previously, the requests could be rejected with `400 Bad Request` response.
+
 ## [v0.28.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v0.28.0-victorialogs)
 
 Released at 2024-07-10