Exporter use a lot of memory when scraping secrets from k8s, which leads to OOM sometimes.

[agrevtsev]

As we are scraping all the secrets from k8s, regardless of their type, it could take a LOT of memory (3G+).
I suppose it could be the reason of #97 #103
For example, long-running cluster can have hundreds of helm.sh/release.v1 secrets per namespace.
So my proposal is fetch only kubeSecretTypes types secrets from k8s API.

I prepared some POC #221 which works.

Disclaimer: unfortunately i'm not proficient in Go programming, so can't program anything complicated yet.

Cheers, Alexey

go tool pprof -sample_index=inuse_space -text heap2.out
File: x509-certificate-exporter
Type: inuse_space
Time: Dec 15, 2023 at 6:40pm (-03)
Showing nodes accounting for 1.54GB, 99.65% of 1.54GB total
Dropped 20 nodes (cum <= 0.01GB)
      flat  flat%   sum%        cum   cum%
    1.54GB 99.65% 99.65%     1.54GB 99.68%  io.ReadAll
         0     0% 99.65%     1.54GB 99.68%  github.com/enix/x509-certificate-exporter/v3/internal.(*Exporter).DiscoverCertificates
         0     0% 99.65%     1.54GB 99.68%  github.com/enix/x509-certificate-exporter/v3/internal.(*Exporter).ListenAndServe
         0     0% 99.65%     1.54GB 99.68%  github.com/enix/x509-certificate-exporter/v3/internal.(*Exporter).getWatchedSecrets
         0     0% 99.65%     1.54GB 99.68%  github.com/enix/x509-certificate-exporter/v3/internal.(*Exporter).parseAllCertificates
         0     0% 99.65%     1.54GB 99.68%  github.com/enix/x509-certificate-exporter/v3/internal.(*Exporter).parseAllKubeSecrets
         0     0% 99.65%     1.54GB 99.68%  k8s.io/client-go/kubernetes/typed/core/v1.(*secrets).List
         0     0% 99.65%     1.54GB 99.68%  k8s.io/client-go/rest.(*Request).Do
         0     0% 99.65%     1.54GB 99.68%  k8s.io/client-go/rest.(*Request).Do.func1
         0     0% 99.65%     1.54GB 99.68%  k8s.io/client-go/rest.(*Request).request
         0     0% 99.65%     1.54GB 99.68%  k8s.io/client-go/rest.(*Request).request.func3
         0     0% 99.65%     1.54GB 99.68%  k8s.io/client-go/rest.(*Request).request.func3.1 (inline)
         0     0% 99.65%     1.54GB 99.68%  k8s.io/client-go/rest.(*Request).transformResponse
         0     0% 99.65%     1.54GB 99.68%  main.main
         0     0% 99.65%     1.54GB 99.68%  runtime.main

[tech1ndex]

+1 seeing the same issue in our environments and have had to bump our memory usage twice already.