fix(cmetrics): prevent appending .0 to scientific notation in bucket_value_to_string

`bucket_value_to_string` used %g to format histogram bucket boundaries,
which switches to scientific notation for values >= 1e6. The subsequent
check for a missing decimal point would then append ".0", producing
malformed labels like "1e+06.0" that Prometheus rejects with:

```
  bucket label "le" is missing or has a malformed value of "1e+08.0"
```

Skip the ".0" suffix when the string already contains 'e', since
scientific notation is valid in Prometheus `le` labels without it.

Fixes #11651

Signed-off-by: Luis Pigueiras <luis.pigueiras@cern.ch>

Author: Pigueiras

lib/cmetrics/src/cmt_encode_prometheus.c

@@ -373,7 +373,11 @@ static cfl_sds_t bucket_value_to_string(double val)
     len = snprintf(str, 64, "%g", val);
     cfl_sds_len_set(str, len);
 
-    if (!strchr(str, '.')) {
+    /*
+     * Append .0 only when there is no decimal point and the number
+     * is not in scientific notation.
+     */
+    if (!strchr(str, '.') && !strchr(str, 'e')) {
         cfl_sds_cat_safe(&str, ".0", 2);
     }