Expose non-nullable metric types

[bogdandrutu]

This is possible because:

• Gogo proto (and protobuf) will not unmarshal any nil element in a oneof;
• Set type will initialize the oneof wrapper as well as the element;

Signed-off-by: Bogdan Drutu bogdandrutu@gmail.com

[codecov]

Codecov Report

Merging #2208 (290bfeb) into master (cb60143) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2208      +/-   ##
==========================================
- Coverage   91.99%   91.92%   -0.08%     
==========================================
  Files         271      271              
  Lines       15779    15653     -126     
==========================================
- Hits        14516    14389     -127     
  Misses        860      860              
- Partials      403      404       +1     

Impacted Files	Coverage Δ
consumer/simple/metrics.go	98.79% <ø> (-0.09%)	⬇️
internal/goldendataset/metric_gen.go	100.00% <ø> (ø)
internal/processor/filterexpr/matcher.go	100.00% <ø> (ø)
...iver/internal/scraper/diskscraper/disk_metadata.go	100.00% <ø> (ø)
...l/scraper/filesystemscraper/filesystem_metadata.go	100.00% <ø> (ø)
...iver/internal/scraper/loadscraper/load_metadata.go	100.00% <ø> (ø)
...nternal/scraper/networkscraper/network_metadata.go	100.00% <ø> (ø)
...nal/scraper/processesscraper/processes_metadata.go	100.00% <ø> (ø)
...nternal/scraper/processscraper/process_metadata.go	100.00% <ø> (ø)
...iver/internal/scraper/swapscraper/swap_metadata.go	100.00% <ø> (ø)
... and 5 more

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