Allow Stale phase in cache_miss

There could be a race condition where a Stale request waiting on cache
lock retries the cache lock and lookup, but enters cache_miss after
finding the asset is no longer present.

.bleep

@@ -1 +1 @@
-0f6ff9925981892f544498c8b322cc66f55281c8
+e68f6024370efed50aebc8741171956acabf9c35

pingora-cache/src/lib.rs

@@ -493,7 +493,8 @@ impl HttpCache {
         match self.phase {
             // from CacheKey: set state to miss during cache lookup
             // from Bypass: response became cacheable, set state to miss to cache
-            CachePhase::CacheKey | CachePhase::Bypass => {
+            // from Stale: waited for cache lock, then retried and found asset was gone
+            CachePhase::CacheKey | CachePhase::Bypass | CachePhase::Stale => {
                 self.phase = CachePhase::Miss;
                 self.inner_mut().traces.start_miss_span();
             }