missing cast to long long in metrics

fixes:
```
core/metrics.c: In function ‘uwsgi_metrics_loop’:
core/metrics.c:393:6: error: format ‘%lld’ expects argument of type ‘long long int’, but argument 4 has type ‘int64_t’ [-Werror=format]
core/metrics.c:393:6: error: format ‘%lld’ expects argument of type ‘long long int’, but argument 4 has type ‘int64_t’ [-Werror=format]
cc1: all warnings being treated as errors
```