fix: tracing not working in mongoose 3.3+

[kjin]

Fixes #1123

Note: We aren't officially adding support for the mongodb npm module (tracing might work, but it almost might not), only extending our support for mongoose.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@259e9f8). Click here to learn what that means.
The diff coverage is 60%.

@@            Coverage Diff            @@
##             master    #1134   +/-   ##
=========================================
  Coverage          ?   94.73%           
=========================================
  Files             ?       99           
  Lines             ?     6511           
  Branches          ?      513           
=========================================
  Hits              ?     6168           
  Misses            ?      181           
  Partials          ?      162

Impacted Files	Coverage Δ
test/plugins/test-trace-mongoose.ts	100% <ø> (ø)
src/config.ts	100% <ø> (ø)
src/plugins/plugin-mongodb.ts	60% <60%> (ø)

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