lib/logger: follow-up for 72f8fce107

- Document the change at docs/CHANELOG.md
- Log fatal errors if the -loggerJSONFields contains unexpected values
- Rename -loggerJsonFields to -loggerJSONFields for the sake of consistency naming commonly used in Go

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2348

docs/CHANGELOG.md

@@ -16,6 +16,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 ## tip
 
 * FEATURE: support overriding of `-search.latencyOffset` value via URL param `latency_offset` when performing requests to [/api/v1/query](https://docs.victoriametrics.com/keyConcepts.html#instant-query) and [/api/v1/query_range](https://docs.victoriametrics.com/keyConcepts.html#range-query). See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3481).
+* FEATURE: allow changing field names in JSON logs if VictoriaMetrics components are started with `-loggerFormat=json` command-line flags. The field names can be changed with the `-loggerJSONFields` command-line flag. For example `-loggerJSONFields=ts:timestamp,msg:message` would rename `ts` and `msg` fields on the output JSON to `timestamp` and `message` fields. See [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2348). Thanks to @michal-kralik for [the pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3488).
 
 * BUGFIX: allow specifying values bigger than 2GiB to the following command-line flag values on 32-bit architectures (`386` and `arm`): `-storage.minFreeDiskSpaceBytes` and `-remoteWrite.maxDiskUsagePerURL`. Previously values bigger than 2GiB were incorrectly truncated on these architectures.
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): stop dropping metric name by a mistake on the [/metric-relabel-debug](https://docs.victoriametrics.com/vmagent.html#relabel-debug) page.

lib/logger/json_fields.go

@@ -0,0 +1,46 @@
+package logger
+
+import (
+	"flag"
+	"log"
+	"strings"
+)
+
+var loggerJSONFields = flag.String("loggerJSONFields", "", `Allows renaming fields in JSON formatted logs. `+
+	`Example: "ts:timestamp,msg:message" renames "ts" to "timestamp" and "msg" to "message". `+
+	"Supported fields: ts, level, caller, msg")
+
+func setLoggerJSONFields() {
+	if *loggerJSONFields == "" {
+		return
+	}
+	fields := strings.Split(*loggerJSONFields, ",")
+	for _, f := range fields {
+		f = strings.TrimSpace(f)
+		v := strings.Split(f, ":")
+		if len(v) != 2 {
+			log.Fatalf("missing ':' delimiter in -loggerJSONFields=%q for %q item", *loggerJSONFields, f)
+		}
+
+		name, value := v[0], v[1]
+		switch name {
+		case "ts":
+			fieldTs = value
+		case "level":
+			fieldLevel = value
+		case "caller":
+			fieldCaller = value
+		case "msg":
+			fieldMsg = value
+		default:
+			log.Fatalf("unexpected json field name in -loggerJSONFields=%q: %q; supported values: ts, level, caller, msg", *loggerJSONFields, name)
+		}
+	}
+}
+
+var (
+	fieldTs     = "ts"
+	fieldLevel  = "level"
+	fieldCaller = "caller"
+	fieldMsg    = "msg"
+)