Update Prometheus docs

docs/prometheus.md

@@ -15,4 +15,7 @@ imgproxy will collect the following metrics:
 * `buffer_size_bytes` - a histogram of the download/gzip buffers sizes (bytes);
 * `buffer_default_size_bytes` - calibrated default buffer size (bytes);
 * `buffer_max_size_bytes` - calibrated maximum buffer size (bytes);
+* `vips_memory_bytes` - libvips memory usage;
+* `vips_max_memory_bytes` - libvips maximum memory usage;
+* `vips_allocs` - the number of active vips allocations;
 * Some useful Go metrics like memstats and goroutines count.