Avoid deadlocking on stateLock in emitMetrics

[ncabatoff]

When stopCh is closed we should stop trying to get the lock. Example deadlock:

POTENTIAL DEADLOCK: 
Previous place where the lock was grabbed 
goroutine 10016 lock 0xc001824a78 
../../init.go:230 vault.(*Core).Initialize { c.stateLock.Lock() } <<<<< 
../../testing.go:335 vault.TestCoreInitClusterWrapperSetup { result, err := core.Initialize(context.Background(), initParams) } 
../../testing.go:1959 vault.(*TestCluster).initCores { bKeys, rKeys, root := TestCoreInitClusterWrapperSetup(t, leader.Core, leader.Handler) } 
../../testing.go:1692 vault.NewTestCluster { testCluster.initCores(t, opts, addAuditBackend) } 
api_integration_test.go:60 api.testVaultServerCoreConfig { cluster := vault.NewTestCluster(t, coreConfig, &vault.TestClusterOptions{ } 
api_integration_test.go:35 api.testVaultServerUnseal { return testVaultServerCoreConfig(t, &vault.CoreConfig{ } 
api_integration_test.go:26 api.testVaultServer { client, _, closer := testVaultServerUnseal(t) } 
secret_test.go:467 api.TestSecret_TokenAccessor.func3 { client, closer := testVaultServer(t) } 
 
Have been trying to lock it again for more than 30s 
goroutine 13525 lock 0xc001824a78 
../../ha.go:58 vault.(*Core).Standby { c.stateLock.RLock() } <<<<< 
../../core_metrics.go:23 vault.(*Core).metricsLoop { if standby, _ := c.Standby(); standby || c.IsDRSecondary() { } 
../../core_metrics.go:314 vault.(*Core).emitMetrics { c.metricsLoop(stopCh) }

[ccapurso]

Looks good to me