module/apmhttp: add status code to client spans

[axw]

Also, sync latest schema.

[codecov-io]

Codecov Report

Merging #388 into master will decrease coverage by 0.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
- Coverage   84.29%   83.96%   -0.34%     
==========================================
  Files         102      102              
  Lines        6253     6136     -117     
==========================================
- Hits         5271     5152     -119     
- Misses        903      913      +10     
+ Partials       79       71       -8

Impacted Files	Coverage Δ
spancontext.go	100% <100%> (ø)	⬆️
model/marshal.go	81.98% <100%> (-4.59%)	⬇️
module/apmhttp/client.go	93.65% <100%> (-0.47%)	⬇️

---

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 c4a71d9...6f29c2b. Read the comment docs.