[RTT] Collect metrics on observed IEEE 802.11mc supporting APs

Collect metrics on the prevalence of support for IEEE 802.11mc by
looking at the number of such APs in each scan.

Bug: 68819947
Test: unit tests
Change-Id: I3857dcfdd43b4ae22a2d2200bfd1f348cbf35a5b
diff --git a/proto/src/wifi.proto b/proto/src/wifi.proto
index 39d0070..9c74188 100644
--- a/proto/src/wifi.proto
+++ b/proto/src/wifi.proto
@@ -382,6 +382,9 @@
 
   // WifiWake statistics
   optional WifiWakeStats wifi_wake_stats = 94;
+
+  // Histogram counting instances of scans with N many 802.11mc (RTT) supporting APs
+  repeated NumConnectableNetworksBucket observed_80211mc_supporting_aps_in_scan_histogram = 95;
 }
 
 // Information that gets logged for every WiFi connection.