feat(healthcheck): HEALTH_TARGET_ADDRESS -> HEALTH_TARGET_ADDRESSES

- Specify fallback addresses
- Defaults changed from cloudflare:443 to cloudflare:443,github.com:443
- Startup check runs a parallel dial to each of the addresses specified with a global 6s timeout
- Full periodic check cycles through addresses as it fails and moves to retry

internal/healthcheck/checker_test.go

@@ -18,11 +18,11 @@ func Test_Checker_fullcheck(t *testing.T) {
 		t.Parallel()
 
 		dialer := &net.Dialer{}
-		const address = "cloudflare.com:443"
+		addresses := []string{"badaddress:9876", "cloudflare.com:443", "google.com:443"}
 
 		checker := &Checker{
-			dialer:      dialer,
-			tlsDialAddr: address,
+			dialer:       dialer,
+			tlsDialAddrs: addresses,
 		}
 
 		canceledCtx, cancel := context.WithCancel(context.Background())
@@ -52,8 +52,8 @@ func Test_Checker_fullcheck(t *testing.T) {
 
 		dialer := &net.Dialer{}
 		checker := &Checker{
-			dialer:      dialer,
-			tlsDialAddr: listeningAddress.String(),
+			dialer:       dialer,
+			tlsDialAddrs: []string{listeningAddress.String()},
 		}
 
 		err = checker.fullPeriodicCheck(ctx)