rgw: add HEAD Object op metrics to perf counters and Prometheus export

rgw: add HEAD Object op metrics to perf counters and Prometheus export

Expose HEAD Object operation metrics in the same format as existing RGW op metrics so operators can track HEAD request volume and latency at gateway, user, and bucket scope.

Previously, HEAD Object requests were silently counted under get_obj_ops because RGWGetObj handles both GET and HEAD. The HEAD-specific early-return path (get_data == false) had no counter instrumentation.

Fixes: ISCE-3042
Signed-off-by: Harsimran Singh <harsimransingh@ibm.com>