v-m/vmagent: Scrape public clouds to test Internet

Scraping the public DNS servers doesn't work anymore since the firewall
routes traffic through Mullvad.  Pinging public cloud providers should
give a pretty decent indication of Internet connectivity.  It will also
serve as a benchmark for the local DNS performance, since the names will
have to be resolved.

victoria-metrics/blackbox.yml

@@ -26,6 +26,8 @@ modules:
   icmp:
     prober: icmp
     timeout: 5s
+    icmp:
+      preferred_ip_protocol: ip4
   smtp:
     prober: tcp
     tcp:

victoria-metrics/scrape.yml

@@ -22,9 +22,10 @@ scrape_configs:
     - icmp
   static_configs:
   - targets:
-    - 1.1.1.1
-    - 8.8.8.8
-    - 9.9.9.9
+    - github.com
+    - cloudflare.com
+    - amazonaws.com
+    - azure.com
   relabel_configs:
   - source_labels: [__address__]
     target_label: __param_target