pings: add Telemetry Gateway IP (#677)

Author: unknwon

docs/admin/pings.mdx

@@ -212,6 +212,7 @@ Sourcegraph aggregates usage and performance metrics for some product features i
 
 Starting 5.2.0:
 - For `pings.sourcegraph.com`, allowlist the IP address: `34.36.231.254`
+- For `telemetry-gateway.sourcegraph.com`, allowlist the IP address: `34.107.157.31`
 - For `sourcegraph.com`, allowlist the full [Cloudflare IP ranges](https://www.cloudflare.com/ips/)
 
 Prior to 5.2.0, allowlist the full [Cloudflare IP ranges](https://www.cloudflare.com/ips/).