fix(kuma-dp): set prometheus content-type when returning metrics

[lukidzi]

Summary

kuma-dp scrapes metrics from the applications, aggregates them, and returns. Unfortunately we were not setting correct content-type and some applications could file while scraping. Added setting of content type based on https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#basic-info. Also, added passing of prometheus requests header to the application. based on the code (https://github.com/prometheus/prometheus/blob/main/scrape/scrape.go#L772)

Checklist prior to review

- [ ] Link to docs PR or issue --
- [ ] Link to UI issue or PR --

• Is the [issue worked on linked][1]? -- Fix Kuma "metrics hijacking"/exposing application metrics not setting Accept header. #4607
  - [ ] Unit Tests --
• E2E Tests --
• Manual Universal Tests --
• Manual Kubernetes Tests --
  - [ ] Do you need to update UPGRADE.md? --
  - [ ] Does it need to be backported according to the backporting policy? --

[codecov-commenter]

Codecov Report

Merging #4706 (9fc314f) into master (d841111) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4706      +/-   ##
==========================================
+ Coverage   46.42%   46.45%   +0.03%     
==========================================
  Files         687      687              
  Lines       46692    46699       +7     
==========================================
+ Hits        21677    21695      +18     
+ Misses      23094    23087       -7     
+ Partials     1921     1917       -4     

Impacted Files	Coverage Δ
app/kuma-dp/pkg/dataplane/metrics/server.go	11.32% <0.00%> (-0.81%)	⬇️
pkg/core/resources/manager/cache.go	88.31% <0.00%> (+2.59%)	⬆️
pkg/api-server/server.go	77.59% <0.00%> (+2.90%)	⬆️
pkg/xds/cache/once/once.go	100.00% <0.00%> (+5.00%)	⬆️
pkg/plugins/leader/postgres/leader_elector.go	89.36% <0.00%> (+6.38%)	⬆️
pkg/xds/cache/once/cache.go	94.87% <0.00%> (+12.82%)	⬆️

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