fix: cast to proper interface

[codyoss]

Fixes #123

[codecov]

Codecov Report

Merging #124 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #124      +/-   ##
============================================
- Coverage     66.32%   66.27%   -0.06%     
- Complexity      365      370       +5     
============================================
  Files            34       34              
  Lines          1877     1877              
  Branches        240      240              
============================================
- Hits           1245     1244       -1     
- Misses          530      531       +1     
  Partials        102      102

Impacted Files	Coverage Δ	Complexity Δ
...src/main/java/com/google/cloud/ServiceOptions.java	43.47% <100%> (-0.4%)	33 <0> (+1)
.../java/com/google/cloud/testing/CommandWrapper.java	87.87% <0%> (-9.1%)	13% <0%> (ø)
...a/com/google/cloud/testing/BaseEmulatorHelper.java	31.42% <0%> (+0.57%)	5% <0%> (ø)	⬇️
.../com/google/cloud/MonitoredResourceDescriptor.java	84.03% <0%> (+1.68%)	24% <0%> (+4%)	⬆️

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