Save metrics to barrier without going through c.systemBarrierView (#884)

* Don't bother trying to save metrics when we don't have a barrier.  Use stateLock.

* Use c.barrier instead of c.systemBarrierView, thus we don't need locking
and don't need to worry about race with mount setup.

* Remove unneccessary lock.

vault/counters.go

@@ -13,7 +13,7 @@ import (
 const (
 	requestCounterDatePathFormat = "2006/01"
 	countersPath                 = systemBarrierPrefix + "counters"
-	requestCountersRelPath       = "counters/requests/"
+	requestCountersPath          = "sys/counters/requests/"
 )
 
 type counters struct {
@@ -51,7 +51,7 @@ type DatedRequestCounter struct {
 // loadAllRequestCounters returns all request counters found in storage,
 // ordered by time (oldest first.)
 func (c *Core) loadAllRequestCounters(ctx context.Context, now time.Time) ([]DatedRequestCounter, error) {
-	view := c.systemBarrierView.SubView(requestCountersRelPath)
+	view := NewBarrierView(c.barrier, requestCountersPath)
 
 	datepaths, err := view.List(ctx, "")
 	if err != nil {
@@ -116,7 +116,7 @@ func (c *Core) loadCurrentRequestCounters(ctx context.Context, now time.Time) er
 // If nothing is found at that path, that isn't an error: a reference to a zero
 // RequestCounter is returned.
 func (c *Core) loadRequestCounters(ctx context.Context, datepath string) (*RequestCounter, error) {
-	view := c.systemBarrierView.SubView(requestCountersRelPath)
+	view := NewBarrierView(c.barrier, requestCountersPath)
 
 	out, err := view.Get(ctx, datepath)
 	if err != nil {
@@ -140,7 +140,7 @@ func (c *Core) loadRequestCounters(ctx context.Context, datepath string) (*Reque
 // we've entered a new month.
 // now should be the current time; it is a parameter to facilitate testing.
 func (c *Core) saveCurrentRequestCounters(ctx context.Context, now time.Time) error {
-	view := c.systemBarrierView.SubView(requestCountersRelPath)
+	view := NewBarrierView(c.barrier, requestCountersPath)
 	requests := atomic.LoadUint64(c.counters.requests)
 	curDatePath := now.Format(requestCounterDatePathFormat)