Stop creating root spans without existing spans

[XSAM]

otelsql will not create root spans without existing spans by default; discrete spans will disappear.

It resolves #7 because sql.conn.reset_session will not occur if there is no parent spans exist.

AllowRoot option is for preventing backward incompatible.

[codecov]

Codecov Report

Merging #13 (0bc9d52) into main (8c94007) will increase coverage by 1.70%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   80.49%   82.20%   +1.70%     
==========================================
  Files          11       11              
  Lines         282      309      +27     
==========================================
+ Hits          227      254      +27     
  Misses         45       45              
  Partials       10       10              

Impacted Files	Coverage Δ
config.go	100.00% <ø> (ø)
conn.go	79.31% <100.00%> (+2.38%)	⬆️
rows.go	56.09% <100.00%> (+4.74%)	⬆️
stmt.go	82.22% <100.00%> (+2.22%)	⬆️
tx.go	100.00% <100.00%> (ø)
utils.go	100.00% <100.00%> (ø)

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