Fix kafka plugin sometimes trace doesn't show span layer

[J-Cod3r]

Please answer these questions before submitting a pull request

• Why submit this pull request?
• Bug fix
• New feature provided
• Improve performance

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Bug fix

• Which version of SkyWalking, OS and JRE?
  SkyWalking 7.0, Redhat 7.6, jre 11
• Bug description.
  kafka plugin sometimes trace doesn't show span layer
  

[wu-sheng]

Why is layer important for a callback?

[codecov]

Codecov Report

Merging #5264 into master will decrease coverage by 0.35%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #5264      +/-   ##
============================================
- Coverage     53.43%   53.08%   -0.36%     
+ Complexity     3172     3167       -5     
============================================
  Files           816      820       +4     
  Lines         20451    20521      +70     
  Branches       1967     1974       +7     
============================================
- Hits          10929    10893      -36     
- Misses         8639     8746     +107     
+ Partials        883      882       -1     

Impacted Files	Coverage Δ	Complexity Δ
...s/manual/database/DatabaseStatementDispatcher.java	10.00% <0.00%> (-90.00%)	1.00% <0.00%> (-1.00%)
...earch7/query/ProfileThreadSnapshotQueryEs7DAO.java	50.00% <0.00%> (-50.00%)	1.00% <0.00%> (-2.00%)
...skywalking/oap/server/core/analysis/topn/TopN.java	0.00% <0.00%> (-33.34%)	0.00% <0.00%> (-1.00%)
...er/core/analysis/data/LimitedSizeBufferedData.java	15.38% <0.00%> (-23.08%)	1.00% <0.00%> (-1.00%)
...ng/oap/server/core/analysis/worker/TopNWorker.java	43.75% <0.00%> (-12.50%)	4.00% <0.00%> (-1.00%)
...rver/core/analysis/worker/TopNStreamProcessor.java	65.38% <0.00%> (-11.54%)	4.00% <0.00%> (-1.00%)
...nalysis/manual/database/TopNDatabaseStatement.java	4.16% <0.00%> (-8.34%)	0.00% <0.00%> (-2.00%)
...rary/client/elasticsearch/ElasticSearchClient.java	54.34% <0.00%> (-4.35%)	25.00% <0.00%> (-1.00%)
...e/parser/listener/MultiScopesAnalysisListener.java	68.33% <0.00%> (-2.23%)	24.00% <0.00%> (ø%)
...ng/oap/server/core/alarm/provider/RunningRule.java	51.85% <0.00%> (-0.53%)	10.00% <0.00%> (ø%)
... and 10 more

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[J-Cod3r]

Why is layer important for a callback?

Not important, but is it unfriendly to display "Unknown"？

[wu-sheng]

Usually, the layer is sensitive for Entry/Exit span only. This is harmless, but at the same time, also meaningless.

[wu-sheng]

With this kind of changes, it affects the plugin test. Please fix them.