PMM-507: Add Prometheus scrape jobs API.

[AlekSi]

No description provided.

[codecov-io]

Codecov Report

Merging #3 into master will decrease coverage by 2.31%.
The diff coverage is 18.19%.

@@           Coverage Diff            @@
##           master     #3      +/-   ##
========================================
- Coverage   12.21%   9.9%   -2.32%     
========================================
  Files          16     20       +4     
  Lines         925   1484     +559     
========================================
+ Hits          113    147      +34     
- Misses        791   1316     +525     
  Partials       21     21

Impacted Files	Coverage Δ
api/alerts.pb.go	5.98% <ø> (ø)	⬆️
cmd/pmm-managed/main.go	0% <0%> (ø)	⬆️
handlers/scrape_jobs.go	0% <0%> (ø)
api/scrape_jobs.pb.gw.go	0% <0%> (ø)
services/prometheus_alerts.go	0% <0%> (-63.94%)	⬇️
api/scrape_jobs.pb.go	5.04% <5.04%> (ø)
services/prometheus.go	32.45% <60.71%> (-21.72%)	⬇️
services/prometheus_scrape_jobs.go	70% <70%> (ø)
... and 4 more

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