fix: metric loss when a previously expired metric is re-added

[shreemaan-abhishek]

Description

A worker-local cache (self.lookup and self.index) was used to optimize metric lookups. However, because ngx.shared.dict automatically evicts keys upon TTL expiration but the local Lua memory cache is not automatically synced to reflect this eviction, a drift occurs:

A metric key expires and is removed from the global ngx.shared.dict.
A worker still holds a reference to this metric in its local self.index cache.
When the worker tries to increment/update this metric, KeyIndex:add() sees the metric in self.index and skips adding it to the shared dictionary.
As a result, the metric is completely lost from the KeyIndex:list() scrape cycle, causing missing metrics in Prometheus.

Solution:

• When a worker looks up a metric to increment/update it, if it has an expiration time set (self.exptime), we ensure the key index relationship is updated via self._key_index:add to put it back into the list.

• regularly sweep the local index and clear out references to keys that have TTL-expired from the global dictionary. This prevents memory leaks in the Lua process and ensures that KeyIndex:list() doesn't iterate over thousands of dead keys.

[shreemaan-abhishek]

@coderabbitai pls review

[membphis]

@shreemaan-abhishek why do we need this PR? can you tell me more about why we do this?

seems relate to apache/apisix#11113

[shreemaan-abhishek]

@membphis yes, you are right.