Set default propagator to noop

[morigs]

Resolves #221

[codecov]

Codecov Report

Merging #329 into master will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   53.27%   53.42%   +0.14%     
==========================================
  Files          71       71              
  Lines        5800     5818      +18     
==========================================
+ Hits         3090     3108      +18     
  Misses       2710     2710              

Impacted Files	Coverage Δ
opentelemetry/src/sdk/propagation/composite.rs	95.45% <100.00%> (+0.71%)	⬆️
...ntelemetry/src/sdk/metrics/aggregators/ddsketch.rs	78.59% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e87c435...1d4000b. Read the comment docs.

[jtescher]

Looks like the CLA bot is stuck again