fix(3.1.x): blanks span for session keepAlive traces

[thiagotnunes]

The keepAlive traces were being tracked along with the parent span set by the client, which clutters the tracing stack. Here, we set the tracing to blank before issuing the keepAlive query.

[codecov]

Codecov Report

Merging #855 (7d897e1) into 3.1.x (0fd859d) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##              3.1.x     #855      +/-   ##
============================================
- Coverage     85.05%   85.05%   -0.01%     
+ Complexity     2556     2554       -2     
============================================
  Files           142      142              
  Lines         13930    13934       +4     
  Branches       1326     1326              
============================================
+ Hits          11848    11851       +3     
+ Misses         1526     1525       -1     
- Partials        556      558       +2     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/spanner/SessionImpl.java	83.52% <0.00%> (-0.50%)	29.00 <0.00> (ø)
...ain/java/com/google/cloud/spanner/SessionPool.java	88.56% <100.00%> (+0.03%)	71.00 <0.00> (ø)
...ud/spanner/SessionPoolAsyncTransactionManager.java	85.36% <0.00%> (-1.63%)	11.00% <0.00%> (-2.00%)
...a/com/google/cloud/spanner/AsyncResultSetImpl.java	91.89% <0.00%> (-0.39%)	32.00% <0.00%> (ø%)
...m/google/cloud/spanner/connection/SpannerPool.java	86.11% <0.00%> (+1.66%)	31.00% <0.00%> (ø%)

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