fix: clear BatchSpanProcessor internal spans buffer before exporting

[TsvetanMilanov]

Which problem is this PR solving?

• This PR solves the problem with the BatchSpanProcessor which exports the same spans multiple times
  in some cases. Fixes BatchSpanProcessor exports spans multiple times #1665

Short description of the changes

• The PR ensures that the internal spans buffer of the BatchSpanProcessor is cleared before another invocation
  of the _flush method is started.

[linux-foundation-easycla]

• ❌ Tsvetan Milanov The commit (84ecf36) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[codecov]

Codecov Report

Merging #1666 (17e0d74) into master (97736be) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1666      +/-   ##
==========================================
- Coverage   91.35%   91.35%   -0.01%     
==========================================
  Files         165      165              
  Lines        5043     5042       -1     
  Branches     1040     1040              
==========================================
- Hits         4607     4606       -1     
  Misses        436      436              

Impacted Files	Coverage Δ
...telemetry-tracing/src/export/BatchSpanProcessor.ts	90.76% <100.00%> (-0.14%)	⬇️

[obecny]

lgtm, thx