Making RPC Upgrade mode reloadable. (#11144)

- Making RPC Upgrade mode reloadable.
- Add suggestions from code review
- remove spurious comment
- switch to require(t,...) form for test.
- Add to changelog

.changelog/11144.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+agent: Added `tls -> rpc_upgrade_mode` to be reloaded on SIGHUP
+```

command/agent/agent.go

@@ -1098,6 +1098,10 @@ func (a *Agent) ShouldReload(newConfig *Config) (agent, http bool) {
 		agent = true
 	}
 
+	if a.config.TLSConfig.RPCUpgradeMode != newConfig.TLSConfig.RPCUpgradeMode {
+		agent = true
+	}
+
 	return agent, http
 }
 

command/agent/agent_test.go

@@ -1296,6 +1296,27 @@ func TestServer_ShouldReload_ShouldHandleMultipleChanges(t *testing.T) {
 	}
 }
 
+func TestServer_ShouldReload_ReturnTrueForRPCUpgradeModeChanges(t *testing.T) {
+	t.Parallel()
+
+	sameAgentConfig := &Config{
+		TLSConfig: &config.TLSConfig{
+			RPCUpgradeMode: true,
+		},
+	}
+
+	agent := NewTestAgent(t, t.Name(), func(c *Config) {
+		c.TLSConfig = &config.TLSConfig{
+			RPCUpgradeMode: false,
+		}
+	})
+	defer agent.Shutdown()
+
+	shouldReloadAgent, shouldReloadHTTP := agent.ShouldReload(sameAgentConfig)
+	require.True(t, shouldReloadAgent)
+	require.False(t, shouldReloadHTTP)
+}
+
 func TestAgent_ProxyRPC_Dev(t *testing.T) {
 	t.Parallel()
 	agent := NewTestAgent(t, t.Name(), nil)