fix specify explicit buckets in example

[hanyuancheung]

Fix: #2419

• Multiple buckets are expected in Prometheus example, some of which are perhaps empty.
• BTW, Change the original example to show how to specify explicit buckets.

[codecov]

Codecov Report

Merging #2493 (c83d041) into main (4b815ff) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #2493   +/-   ##
=====================================
  Coverage   76.3%   76.3%           
=====================================
  Files        173     173           
  Lines      12022   12022           
=====================================
+ Hits        9175    9182    +7     
+ Misses      2602    2597    -5     
+ Partials     245     243    -2     

Impacted Files	Coverage Δ
sdk/metric/sdk.go	81.5% <0.0%> (+1.4%)	⬆️
sdk/metric/refcount_mapped.go	100.0% <0.0%> (+20.0%)	⬆️