glog: add JSON structured logging mode (#8708)

* glog: add JSON structured logging mode

Add opt-in JSON output format for glog, enabling integration with
log aggregation systems like ELK, Loki, and Datadog.

- Add --log_json flag to enable JSON output at startup
- Add SetJSONMode()/IsJSONMode() for runtime toggle
- Add JSON branches in println, printDepth, printf, printWithFileLine
- Use manual JSON construction (no encoding/json) for performance
- Add jsonEscapeString() for safe string escaping
- Include 8 unit tests and 1 benchmark

Enabled via --log_json flag. Default behavior unchanged.

* glog: prevent lazy flag init from overriding SetJSONMode

If --log_json=true and SetJSONMode(false) was called at runtime,
a subsequent IsJSONMode() call would re-enable JSON mode via the
sync.Once lazy initialization. Mark jsonFlagOnce as done inside
SetJSONMode so the runtime API always takes precedence.

* glog: fix RuneError check to not misclassify valid U+FFFD

The condition r == utf8.RuneError matches both invalid UTF-8
sequences (size=1) and a valid U+FFFD replacement character
(size=3). Without checking size == 1, a valid U+FFFD input
would be incorrectly escaped and only advance by 1 byte,
corrupting the output.

---------

Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/glog/glog_file.go

@@ -64,6 +64,10 @@ var logMaxFiles = flag.Int("log_max_files", 5, "Maximum number of log files to k
 // The default is 168 hours (7 days). Set to 0 to disable time-based rotation.
 var logRotateHours = flag.Int("log_rotate_hours", 168, "Rotate log files after this many hours (default: 168 = 7 days, 0 = disabled)")
 
+// logJSON enables JSON-formatted log output (one JSON object per line).
+// Useful for integration with ELK, Loki, Datadog, and other log aggregation systems.
+var logJSON = flag.Bool("log_json", false, "Output logs in JSON format instead of glog text format")
+
 // logCompress enables gzip compression of rotated log files.
 // Compressed files get a .gz suffix. Compression runs in the background.
 var logCompress = flag.Bool("log_compress", false, "Gzip-compress rotated log files to save disk space")