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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