Add tracing support for .count() and .countDocuments() in datadog-plugin-mongodb-core

[patrickshuff]

Currently there are only a limited number of mongodb-core calls that are being dynamically patched:
https://github.com/DataDog/dd-trace-js/blob/master/packages/datadog-plugin-mongodb-core/src/index.js#L139

function patch (core, tracer, config) {
  this.wrap(core.Server.prototype, 'command', createWrapOperation(tracer, config))
  this.wrap(core.Server.prototype, 'insert', createWrapOperation(tracer, config, 'insert'))
  this.wrap(core.Server.prototype, 'update', createWrapOperation(tracer, config, 'update'))
  this.wrap(core.Server.prototype, 'remove', createWrapOperation(tracer, config, 'remove'))

  if (core.Cursor.prototype.next) {
    this.wrap(core.Cursor.prototype, 'next', createWrapNext(tracer, config))
  } else if (core.Cursor.prototype._next) {
    this.wrap(core.Cursor.prototype, '_next', createWrapNext(tracer, config))
  }
}

Please add support for the default patching/tracing of both the .count() and the .countDocument() queries as they (unfortunately) can be very expensive if there are MongoDB indexes that are missing and are worth tracing by default.

[rochdev]

Can you provide a code example of how you use count()? I tried reproducing but I'm getting traces for count commands.

[rochdev]

We're looking into instrumenting the driver at the wire protocol directly which should intercept every command sent to Mongo, so this will be supported as well at that point.

[patrickshuff]

@rochdev -- we are using mongoose-paginate-v2 which was doing some .count() calls and were causing us some pain that weren't showing up in Datadog APM.

[rochdev]

Fixed in #1159