Set default metrics interval to 30s

[axw]

Closes #315

[codecov-io]

Codecov Report

Merging #322 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
- Coverage    80.9%   80.87%   -0.03%     
==========================================
  Files          96       96              
  Lines        5766     5774       +8     
==========================================
+ Hits         4665     4670       +5     
  Misses        837      837              
- Partials      264      267       +3

Impacted Files	Coverage Δ
env.go	71.76% <ø> (ø)	⬆️
tracer.go	84.32% <66.66%> (-0.38%)	⬇️

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