simpler custom spawn bucket sizes

for prometheus

docs/source/reference/monitoring.md

@@ -33,6 +33,16 @@ export JUPYTERHUB_METRICS_PREFIX=jupyterhub_prod
 
 would result in the metric `jupyterhub_prod_active_users`, etc.
 
+## Customizing spawn bucket sizes
+
+As of JupyterHub >= 5.2.1, override `JUPYTERHUB_SERVER_SPAWN_DURATION_SECONDS_BUCKETS` env variable in Hub's environment to allow custom bucket sizes. Otherwise default to, [0.5, 1, 2.5, 5, 10, 15, 30, 60, 120, 180, 300, 600, float("inf")]
+
+For example,
+
+```bash
+export JUPYTERHUB_SERVER_SPAWN_DURATION_SECONDS_BUCKETS="1,2,4,6,12,30,60,120"
+```
+
 ## Configuring metrics
 
 ```{eval-rst}