metrics: add link to OSBv1

Author: sanjayankur31

source/General/Metrics.md

@@ -5,6 +5,8 @@ The plot shows the trend in number of OSBv2 users over time.
 It is periodically updated using the script [here](https://github.com/OpenSourceBrain/Documentation/blob/main/scripts/get-user-metrics.py).
 Please note that this script can only be run by OSBv2 administrators as it requires access to the user database that non-admin users will not have.
 
+For usage metrics for OSBv1, please refer to [this page](https://v1.opensourcebrain.org/projects#people).
+
 ```{figure} ../images/20250528-osb-users-plot.png
 :alt: Plot showing number of OSBv2 users over time
 :align: center