perf(plugin/prometheus): generate metrics output data with string.buffer

[chronolaw]

Summary

This PR follows #11040, continue to optimize the string usage by string.buffer.

KAG-1852

Checklist

• The Pull Request has tests
• There's an entry in the CHANGELOG
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• use table.new for seen_metrics
• buffered_print() outputs a string, not a table
• change printable_metric_data of stream to fit the new buffered_print()
• do not use ipairs to iterate array
• other small optimizations

[chronolaw]

@oowl , could you do the same performance test for this PR again? thanks.

[oowl]

LGTM, I think maybe we can contribute these performances related patches to https://github.com/knyar/nginx-lua-prometheus upstream, because prometheus.lua was a fork with upstream.

[oowl]

@oowl , could you do the same performance test for this PR again? thanks.

Sorry, performance test environment was destroy in last month

[chronolaw]

LGTM, I think maybe we can contribute these performances related patches to https://github.com/knyar/nginx-lua-prometheus upstream, because prometheus.lua was a fork with upstream.

Sure, Do we have someone responsible for tracking or syncing the nginx-lua-prometheus upstream?