Add __printf attribute to homa_metric_append

* Lets the compiler check the format strings passed by every caller
  (none are wrong today) and clears gcc W=1's
  -Wsuggest-attribute=format warning in homa_metrics.c.

Resolves #125.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: 2 people

homa_metrics.h

@@ -869,7 +869,8 @@ static inline struct homa_metrics *homa_metrics_per_cpu(void)
 
 extern struct homa_metrics_output homa_mout;
 
-void     homa_metric_append(const char *name, u64 value, const char *format, ...);
+void     homa_metric_append(const char *name, u64 value,
+			    const char *format, ...) __printf(3, 4);
 void     homa_metrics_end(void);
 int      homa_metrics_init(void);
 loff_t   homa_metrics_lseek(struct file *file, loff_t offset,