feat(prometheus): Include tags in metrics labels

[carnei-ro]

Moving Kong/kong-plugin-prometheus#149 to here

closes #7678

[hbagdi]

I'm not sure if the sorting order of tags in Kong's DAO is stable or not i.e. can the order of tags change or not.
If it can change, I'd recommend sorting them because otherwise, it will lead to unnecessary duplication of metrics in the Prometheus store.

This is also additional table pollution probably. Wangchong will know best how to tackle that part.

[carnei-ro]

Right! I added sorting 😄

[hbagdi]

Please add a test case for when there are tags on the service or route. Please make sure to have multiple tags to test for sort order.

[carnei-ro]

I wrote all those tests at 02-access_spec.lua, there is a scenario for: exposing all tags, only services and only route. Do you think I should replicate it in here?

[rmrf]

how this going

[mayocream]

@carnei-ro hi, any updates?

[carnei-ro]

hey guys, I'm not really sure why some tests are failing - need help

[mayocream]

Hi @carnei-ro

IMO, I don't think this behavior should be included in promethues plugin officially.

1. We might want to avoid adding empty labels like service_tags="",route_tags="" by default
2. Don't overuse labels, cardinality can be very large

ref:

• Add Consul Tags to labels prometheus/consul_exporter#45

[hbagdi]

We won't be able to accept this contribution at this point. The cardinality issues in the plugin have been plaguing Kong's performance for quite some time. Please revisit this change in 6 months.