Add more InstanaExporterHandler tests

[augustjanse]

Four tests for InstanaExporterHandler. These hit four previously uncovered branches in convertToJson(), namely the if statements that check for:

• null status
• Error status
• Multiple spans in a collection
• Multiple attributes in a span

[codecov-io]

Codecov Report

Merging #2014 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #2014 +/- ## ============================================ - Coverage 83.13% 83.12% -0.01%  Complexity 2250 2250 ============================================ Files 319 319 Lines 10527 10527 Branches 1047 1047 ============================================ - Hits 8752 8751 -1  Misses 1437 1437 - Partials 338 339 +1

Impacted Files	Coverage Δ	Complexity Δ
...re/trace/export/InProcessSampledSpanStoreImpl.java	95.85% <0%> (-0.6%)	22% <0%> (ø)

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[songy23]

LGTM