feat: add tracestate header when starting lambda transaction

[astorm]

Checklist

Part of #2156

This PR ensures that if the Lambda function is responding to an API Gateway request (or other request that includes a tracestate header) that its transaction will start using that tracestate. This is needed because our existing lambda instrumentation was written before tracestate was a thing.

• Implement code
• Add tests

[apmmachine]

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• Start Time: 2021-08-10T21:16:41.329+0000

• Duration: 19 min 49 sec

• Commit: 5f8c41e

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[trentm]

1. The handling will have to change to avoid that break preferring 'elastic-apm-traceparent' to 'traceparent'. IIUC, with the current logic it will miss tracestate if it comes after 'elastic-apm-traceparent' in payload.headers.
2. Also, per feat: prefer 'traceparent' header over 'elastic-apm-traceparent' header #2079 the logic should change (could be a separate PR) to prefer 'traceparent' to 'elastic-apm-traceparent'.

[astorm]

@trentm Ugh -- that's a weird construct. Thanks for catching it. Any strong preference between

• more code and fewer loop cycles (matching most closely with current intent/pattern)

    if (payload.headers !== undefined) {
      for (const [key, value] of Object.entries(payload.headers)) {
        const lowerCaseKey = key.toLowerCase()
        if (lowerCaseKey === 'traceparent') {
          parentId = value
        } else if (!parentId && lowerCaseKey === 'elastic-apm-traceparent') {
          parentId = value
        } else if(lowerCaseKey === 'tracestate') {
          tracestate = value
        }
        if(parentId && tracestate) {
          break
        }
      }
    }

• or less code but slightly more work normalizing all the headers

      if (payload.headers !== undefined) {
        const normalizedHeaders = {}
        for (const [key, value] of Object.entries(payload.headers)) {
          const lowerCaseKey = key.toLowerCase()
          normalizedHeaders[lowerCaseKey] = value
        }
        parentId = normalizedHeaders.traceparent ? normalizedHeaders.traceparent : normalizedHeaders['elastic-apm-traceparent']
        tracestate = normalizedHeaders.tracestate
      }

[trentm]

The former will prefer 'elastic-apm-traceparent' over 'traceparent' if it and 'tracestate' come first, so doesn't work as written.

No strong preference between those two until perf profiles show that the section is a hot spot.

Slightly related: from old https://elastic.slack.com/archives/C5M02D1GE/p1619802623086700?thread_ts=1619742610.083400&cid=C5M02D1GE for ([key, value] of Object.entries(obj)) is dog slow. Perhaps premature optimization, but I'd generally bias against using it.

[astorm]

I've replaced the Object.entries() loop with an Object.keys() loop. Always up for quashing these then we see them and remember.

I've opted for the clearer (to me) code of normalizing all the headers rather than the micro-optimization of finishing once we have the specific headers we're interested in.

I've added test cases with both elastic-apm-traceparent by itself and elastic-apm-traceparent both before and after the traceparent header to ensure that we prefer traceparent when it's there, but still pickup elastic-apm-traceparent when it's by itself.

[trentm]

Thanks.