Fix metrics when cache miss

[jja725]

What changes are proposed in this pull request?

Fix metrics when there's cache miss

Why are the changes needed?

The correct behavior should be

1. When read from external storage (or a cache miss?),
   CLIENT_CACHE_PAGE_READ_CACHE_TIME_NS == 0 & CLIENT_CACHE_PAGE_READ_EXTERNAL_TIME_NS > 0

2. When read from cache (or cache hit?),
   CLIENT_CACHE_PAGE_READ_CACHE_TIME_NS > 0 & CLIENT_CACHE_PAGE_READ_EXTERNAL_TIME_NS == 0

Does this PR introduce any user facing changes?

na

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