feat(http, metrics): use metrics as middleware

[rustatian]

Reason for This PR

ref: #116 (partly)

Description of Changes

• Move metrics to the separate middleware. Prepare the base for the extended http metrics.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.] [Reviewer TODO: Verify that these criteria are met. Request changes if not]

• [x] All commits in this PR are signed (git commit -s).
• [x] The reason for this PR is clearly provided (issue no. or explanation).
• [x] The description of changes is clear and encompassing.
• [x] Any required documentation changes (code and docs) are included in this PR.
• [x] Any user-facing changes are mentioned in CHANGELOG.md.
• [x] All added/changed functionality is tested.

[codecov[bot]]

Codecov Report

Merging #122 (251efce) into master (f6db990) will decrease coverage by 0.00%. The diff coverage is 92.95%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
- Coverage   65.29%   65.29%   -0.01%     
==========================================
  Files         134      135       +1     
  Lines       10780    10822      +42     
==========================================
+ Hits         7039     7066      +27     
- Misses       3036     3048      +12     
- Partials      705      708       +3     

Impacted Files	Coverage Δ
http/middleware/prometheus/writer.go	71.42% <ø> (ø)
http/middleware/prometheus/plugin.go	92.53% <92.53%> (ø)
http/metrics.go	100.00% <100.00%> (ø)
http/plugin.go	76.97% <100.00%> (-1.33%)	:arrow_down:
redis/pubsub/channel.go	81.25% <0.00%> (-6.25%)	:arrow_down:
redis/pubsub/pubsub.go	72.86% <0.00%> (-2.33%)	:arrow_down:
amqp/amqpjobs/item.go	73.28% <0.00%> (-1.37%)	:arrow_down:

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