Fix sporadic test failure in otlp exporter http client tests

[MrAlias]

Apply fix from #1906

Remove the situation where the server hold the processor long enough to timeout before the client by instead using a chan to delay the server that is only closed after the client has timed out.

Fix #2495

[codecov]

Codecov Report

Merging #2496 (15ccfe8) into main (1c1e846) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #2496   +/-   ##
=====================================
  Coverage   76.3%   76.3%           
=====================================
  Files        173     173           
  Lines      12022   12022           
=====================================
+ Hits        9175    9179    +4     
+ Misses      2602    2598    -4     
  Partials     245     245           

Impacted Files	Coverage Δ
sdk/trace/batch_span_processor.go	85.4% <0.0%> (+2.0%)	⬆️

[hanyuancheung]

Very fast response for the bugfix. Thank you @MrAlias !