fix: keep the error counter when a worker initializes again

[bwangll]

Problem

Prometheus.init() runs in every init_worker, including when nginx respawns a crashed worker. It sets nginx_metric_errors_total to 0 with dict:set, so a respawn wipes the errors already counted by all workers, and the counter goes backwards.

Fix

Create the counter with dict:add, which leaves an existing value untouched.

Tests

TestPrometheus:testInitKeepsErrorCount initializes the library again on a dict that already holds an error count and checks the count is kept. It fails without the fix.

[coderabbitai]

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