module/apmsql: don't report driver.ErrBadConn

[axw]

database/sql drivers may return driver.ErrBadConn
to indicate that the connection is bad, and should
not be reused. We should not report these errors,
as they are expected by database/sql, and used for
connection pooling/reuse.

Closes #345

[codecov-io]

Codecov Report

Merging #346 into master will increase coverage by 3.47%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
+ Coverage   80.14%   83.61%   +3.47%     
==========================================
  Files          98       98              
  Lines        6008     6012       +4     
==========================================
+ Hits         4815     5027     +212     
+ Misses        923      921       -2     
+ Partials      270       64     -206

Impacted Files	Coverage Δ
module/apmsql/conn.go	65.3% <100%> (+6.35%)	⬆️
modelwriter.go	94.16% <0%> (+0.83%)	⬆️
internal/radix/radix.go	39.92% <0%> (+0.92%)	⬆️
utils.go	80.68% <0%> (+1.13%)	⬆️
internal/sqlscanner/scanner.go	96.82% <0%> (+1.58%)	⬆️
metrics.go	91.66% <0%> (+1.66%)	⬆️
error.go	76.36% <0%> (+1.81%)	⬆️
context.go	94.05% <0%> (+1.98%)	⬆️
internal/apmhttputil/url.go	97.87% <0%> (+2.12%)	⬆️
module/apmsql/stmt.go	63.82% <0%> (+2.12%)	⬆️
... and 33 more

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