investigate trace-context propagation after transaction_max_spans is hit

[trentm]

Before this PR that was part of adding transaction.dropped_spans_stats support, the creation of a span that overshot transactionMaxSpans would return null. After that change, a Span instance is returned (though it has .isRecorded() false -- where "recorded" here corresponds to the sampled flag on that span's traceparent being false).

Looking at trace-context propagation for outgoing HTTP requests:
https://github.com/elastic/apm-agent-nodejs/blob/v3.42.0/lib/instrumentation/http-shared.js#L209-L225
there are two impacts of the above change:

1. We will be propagating a traceparent with the sampled flag set false.
2. We will be propagating a traceparent with the parent-id set to the ID of the dropped span.

The latter would cause a broken trace, but given the former, we wouldn't typically expect a downstream service to continue the trace anyway. Before the change (to no longer return null from Transaction.prototype.createSpan()) the APM agent would use the trace-context details from whatever parent span (or the transaction) that was recorded -- so we would get a continued trace.

The same issue will exist for SQS trace-context propagation after the coming change in #3044.

[trentm]

Note to self: Here is an addition to "sqs.test.js" that starts a test case for this.

sqs.test.js snippet

  test.test('API: sendMessage, hit transactionMaxSpans', function (t) {
    const app = createMockServer(
      getXmlResponse('sendMessage')
    )
    const listener = app.listen(0, function () {
      const oldTransactionMaxSpans = agent._conf.transactionMaxSpans
      agent._conf.transactionMaxSpans = 1
      t.on('end', () => {
        agent._conf.transactionMaxSpans = oldTransactionMaxSpans
      })

      resetAgent(function (data) {
        console.log('XXX data:'); console.dir(data, { depth: 5 })
        // t.equals(data.spans.length, 1, 'generated one span')
        // const spanSqs = data.spans[0]
        // t.equals(spanSqs.name, 'SQS SEND to our-queue', 'SQS span named correctly')
        // t.equals(spanSqs.type, 'messaging', 'span type set to messaging')
        // t.equals(spanSqs.subtype, 'sqs', 'span subtype set to sqs')
        // t.equals(spanSqs.action, 'send', 'span action matches API method called')
        // t.deepEqual(spanSqs.context.service.target,
        //   { type: 'sqs', name: 'our-queue' },
        //   'span.context.service.target')
        // t.deepEqual(spanSqs.context.destination, {
        //   address: 'sqs.us-west.amazonaws.com',
        //   port: 443,
        //   cloud: { region: 'us-west' },
        //   service: { type: '', name: '', resource: 'sqs/our-queue' }
        // }, 'span.context.destination')
        // t.equals(spanSqs.context.message.queue.name, 'our-queue', 'queue name context set')

        // Ensure the request sent to SQS included trace-context in message
        // attributes.  The mock server parses the sent urlencoded body into an
        // object on req.body.
        t.equal(app._receivedReqs.length, 1)
        console.log('XXX _receivedReqs:'); console.dir(app._receivedReqs, { depth: 5 })
        const req = app._receivedReqs[0]
        t.equal(req.body.Action, 'SendMessage', 'req.body.Action')
        // The fixture has 3 message attributes, so traceparent and tracestate
        // should be attributes 4 and 5.
        t.equal(req.body['MessageAttribute.4.Name'], 'traceparent', 'traceparent message attribute Name')
        t.equal(req.body['MessageAttribute.4.Value.DataType'], 'String', 'traceparent message attribute DataType')
        t.equal(req.body['MessageAttribute.4.Value.StringValue'], `00-${spanSqs.trace_id}-${spanSqs.id}-01`, 'traceparent message attribute StringValue')
        t.equal(req.body['MessageAttribute.5.Name'], 'tracestate', 'tracestate message attribute Name')
        t.equal(req.body['MessageAttribute.5.Value.DataType'], 'String', 'tracestate message attribute DataType')
        t.equal(req.body['MessageAttribute.5.Value.StringValue'], 'es=s:1', 'tracestate message attribute StringValue')

        t.end()
      })

      const t0 = agent.startTransaction('t0')
      const s1 = t0.startSpan('s1')
      console.log('XXX s1: ', s1.id, s1.name, s1.ended, s1.traceparent)
      setImmediate(() => {
        const s2 = t0.startSpan('s2')
        console.log('XXX s2: ', s2.id, s2.name, s2.ended, s2.traceparent)
        setImmediate(() => {
          const sqs = new AWS.SQS({ apiVersion: '2012-11-05' })
          const params = getParams('sendMessage', listener.address().port)
          sqs.sendMessage(params, function (err, data) {
            t.error(err)
            s2.end()
            s1.end()
            t0.end()
            listener.close()
          })
        })
      })
    })
  })