Apply default buckets in Prometheus Factory

[moraitisk]

Signed-off-by: Konstantinos Moraitis k.moraitis9@gmail.com

Which problem is this PR solving?

• Resolves Define default buckets for Prometheus Histogram #89

Short description of the changes

• The prometheus factory determines which buckets to use when creating a Histogram.
  If the histogram options, provided as arguments, are empty, then the factory uses the default buckets.

[codecov]

Codecov Report

Merging #90 (d6def10) into master (78a0e7f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #90   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines          709       713    +4     
=========================================
+ Hits           709       713    +4     

Impacted Files	Coverage Δ
metrics/prometheus/factory.go	100.00% <100.00%> (ø)

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