Always fall back to string payload if we can't handle structured logs

[dashpole]

Fixes #689

Supersedes #689

This is slightly broader in scope than #689. That PR only handled the case where we are unable to marshal the payload to JSON. It could still fail to export for other reasons, such as map keys or values containing invalid UTF-8. It could also fail if we didn't support the ValueType (e.g. ValueTypeSlice, or ValueTypeInt).

This PR changes the logs exporter to always fall back to attempting to send a raw string payload if we are unable to create a structpb.Struct from the body.

[codecov]

Codecov Report

Merging #701 (f7bde61) into main (0c4b0bb) will increase coverage by 0.32%.
Report is 1 commits behind head on main.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##             main     #701      +/-   ##
==========================================
+ Coverage   69.07%   69.39%   +0.32%     
==========================================
  Files          36       36              
  Lines        4740     4735       -5     
==========================================
+ Hits         3274     3286      +12     
+ Misses       1316     1298      -18     
- Partials      150      151       +1     

Files Changed	Coverage Δ
exporter/collector/integrationtest/diff.go	100.00% <ø> (ø)
...er/collector/integrationtest/inmemoryocexporter.go	87.61% <ø> (ø)
...er/collector/integrationtest/protos/fixtures.pb.go	42.48% <ø> (ø)
...er/collector/integrationtest/testcases/testcase.go	81.51% <ø> (ø)
exporter/collector/logs.go	44.97% <86.66%> (+3.92%)	⬆️

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