Problem

When metrics are registered with an exptime (e.g. expire=300), the KeyIndex
index entries (__key:N) are stored with the same exptime via dict:add().

While metric value keys have their TTL refreshed on every incr/set call,
the KeyIndex index entries are never refreshed because prometheus.lua's
lookup_or_create() returns early from its local Lua cache on cache hit,
skipping the key_index:add() call entirely:

-- prometheus.lua lookup_or_create()
local full_name = t[LEAF_KEY]
if full_name then
    return full_name  -- cache hit: key_index:add() is never called
end

After exptime seconds, the index entry expires and gets removed by
flush_expired() — even for metrics that are still actively being recorded.
On the next metric_data() collection, sync_expired() detects the missing
index entry and removes it from memory, causing the metric to silently
disappear from Prometheus output.

Note on existing TTL refresh code

The current code already attempts to refresh TTL for existing keys (lines 118-133):

if self.index[key] ~= nil then
    if exptime then
        local ok = self.dict:expire(...)
    end
end

However, this code is unreachable for active metrics because lookup_or_create()'s
local cache prevents key_index:add() from being called in the first place.

Fix

Store KeyIndex entries with nil exptime (permanent). Since dict:add() only
succeeds once per key (returns 'exists' error if the key already exists),
changing exptime to nil here means the index entry is created permanently,
while metric value keys still expire normally via incr/set.

flush_expired() will then only remove expired metric value entries, leaving
the index intact — making it safe to call flush_expired() periodically
to reclaim nginx slab memory from expired metrics.

Reproduction

1. Register a metric with exptime=300
2. Send requests so the metric is actively recorded
3. Wait 300 seconds without restarting nginx
4. Call ngx.shared["prometheus-metrics"]:flush_expired()
5. Observe: the metric disappears from /metrics output despite still being active

Summary by CodeRabbit

Release Notes

• Bug Fixes
  • Fixed key expiration handling to ensure proper TTL management when adding new cache entries.