app/{vminsert,vmagent}: add healthcheck for influx ingestion endpoints (#6749)

### Describe Your Changes

This is useful for clients which validate InfluxDB is available before
data ingestion can be started.

See: https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6653

### 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>
Signed-off-by: hagen1778 <roman@victoriametrics.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>

app/vmagent/main.go

@@ -318,6 +318,10 @@ func requestHandler(w http.ResponseWriter, r *http.Request) bool {
 		influxQueryRequests.Inc()
 		influxutils.WriteDatabaseNames(w)
 		return true
+	case "/influx/health":
+		influxHealthRequests.Inc()
+		influxutils.WriteHealthCheckResponse(w)
+		return true
 	case "/opentelemetry/api/v1/push", "/opentelemetry/v1/metrics":
 		opentelemetryPushRequests.Inc()
 		if err := opentelemetry.InsertHandler(nil, r); err != nil {
@@ -564,6 +568,10 @@ func processMultitenantRequest(w http.ResponseWriter, r *http.Request, path stri
 		influxQueryRequests.Inc()
 		influxutils.WriteDatabaseNames(w)
 		return true
+	case "influx/health":
+		influxHealthRequests.Inc()
+		influxutils.WriteHealthCheckResponse(w)
+		return true
 	case "opentelemetry/api/v1/push", "opentelemetry/v1/metrics":
 		opentelemetryPushRequests.Inc()
 		if err := opentelemetry.InsertHandler(at, r); err != nil {
@@ -675,6 +683,7 @@ var (
 	influxWriteErrors   = metrics.NewCounter(`vmagent_http_request_errors_total{path="/influx/write", protocol="influx"}`)
 
 	influxQueryRequests  = metrics.NewCounter(`vmagent_http_requests_total{path="/influx/query", protocol="influx"}`)
+	influxHealthRequests = metrics.NewCounter(`vmagent_http_requests_total{path="/influx/health", protocol="influx"}`)
 
 	datadogv1WriteRequests = metrics.NewCounter(`vmagent_http_requests_total{path="/datadog/api/v1/series", protocol="datadog"}`)
 	datadogv1WriteErrors   = metrics.NewCounter(`vmagent_http_request_errors_total{path="/datadog/api/v1/series", protocol="datadog"}`)

app/vminsert/main.go

@@ -218,6 +218,10 @@ func RequestHandler(w http.ResponseWriter, r *http.Request) bool {
 		addInfluxResponseHeaders(w)
 		influxutils.WriteDatabaseNames(w)
 		return true
+	case "/influx/health":
+		influxHealthRequests.Inc()
+		influxutils.WriteHealthCheckResponse(w)
+		return true
 	case "/opentelemetry/api/v1/push", "/opentelemetry/v1/metrics":
 		opentelemetryPushRequests.Inc()
 		if err := opentelemetry.InsertHandler(r); err != nil {
@@ -398,6 +402,7 @@ var (
 	influxWriteErrors   = metrics.NewCounter(`vm_http_request_errors_total{path="/influx/write", protocol="influx"}`)
 
 	influxQueryRequests  = metrics.NewCounter(`vm_http_requests_total{path="/influx/query", protocol="influx"}`)
+	influxHealthRequests = metrics.NewCounter(`vm_http_requests_total{path="/influx/health", protocol="influx"}`)
 
 	datadogv1WriteRequests = metrics.NewCounter(`vm_http_requests_total{path="/datadog/api/v1/series", protocol="datadog"}`)
 	datadogv1WriteErrors   = metrics.NewCounter(`vm_http_request_errors_total{path="/datadog/api/v1/series", protocol="datadog"}`)

docs/CHANGELOG.md

@@ -30,6 +30,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 
 ## tip
 
+* FEATURE: add `/influx/health` health-check handler for Influx endpoints. This is needed as some clients use the health endpoint to determine if the server is healthy and ready for data ingestion. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6653) for the details.
 * FEATURE: [vmctl](https://docs.victoriametrics.com/vmctl/): add `--vm-backoff-retries`, `--vm-backoff-factor`, `--vm-backoff-min-duration` and `--vm-native-backoff-retries`, `--vm-native-backoff-factor`, `--vm-native-backoff-min-duration` command-line flags. These flags allow to change backoff policy config for import requests to VictoriaMetrics. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6622).
 
 ## [v1.102.1](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.102.1)

lib/influxutils/influxutils.go

@@ -27,3 +27,14 @@ func WriteDatabaseNames(w http.ResponseWriter) {
 	}
 	fmt.Fprintf(w, `{"results":[{"statement_id":0,"series":[{"name":"databases","columns":["name"],"values":[%s]}]}]}`, strings.Join(dbs, ","))
 }
+
+// WriteHealthCheckResponse writes response for influx ping to w.
+func WriteHealthCheckResponse(w http.ResponseWriter) {
+	// Emulate fake response for influx ping.
+	// This is needed for some clients to detect whether InfluxDB is available.
+	// See:
+	// - https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6653
+	// - https://docs.influxdata.com/influxdb/v2/api/#operation/GetHealth
+
+	fmt.Fprintf(w, `{"name":"influxdb", "message":"ready for queries and writes", "status":"pass", "checks":[]}`)
+}