Add support for tracing

[nhinsch]

What does this PR do?

Adds support for tracing.

• Include the dd-trace-go package
• Add two new env vars: DD_TRACE_ENABLED and DD_MERGE_XRAY_TRACES (both default to false)
• If DD_TRACE_ENABLED == true, create a function execution span representing the Lambda function execution and add appropriate tags.
• If the Lambda's trace context came from an event payload OR the user has set DD_MERGE_XRAY_TRACES == true, make the function execution span a child of the current span. In other words, do NOT make the function execution span a child of the current span if the current span is an X-Ray segment and the user does NOT want to merge their Datadog and X-Ray traces.
• Store the source of a trace context when we receive it (necessary for the X-Ray merging logic)
• Rename the ExtractTraceContext method to ContextWithTraceContext to better reflect what it does (it adds the trace context to the context object)
• Deprecate the old method of getting HTTP headers for propagating trace context.
• Update README accordingly

Motivation

Customers want to instrument Go Lambda functions for use with Datadog APM.

Testing Guidelines

Integration tests and manual testing.

Types of changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Checklist

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [x] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-io]

Codecov Report

Merging #44 into master will decrease coverage by 1.92%. The diff coverage is 24.56%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   84.44%   82.52%   -1.93%     
==========================================
  Files          12       12              
  Lines         540      555      +15     
==========================================
+ Hits          456      458       +2     
- Misses         63       81      +18     
+ Partials       21       16       -5     

Impacted Files	Coverage Δ
internal/trace/listener.go	0.00% <0.00%> (ø)
ddlambda.go	80.00% <100.00%> (+3.72%)	:arrow_up:
internal/trace/context.go	87.15% <100.00%> (+0.24%)	:arrow_up:
internal/metrics/processor.go	93.75% <0.00%> (-3.13%)	:arrow_down:
internal/metrics/listener.go	96.29% <0.00%> (+23.79%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e683acf...4ce82c5. Read the comment docs.

[xrn]

Hi @nhinsch @DarcyRaynerDD

I would like to ask, are you sure that this is working? From tests, I see that it is generating trace_id as hex

{
    "traces": [
        [
            {
                "trace_id": "6fb744bf13869d00",
                "span_id": "1e8538262a43c188",
                "parent_id": "1e5df70157538a8e",
                "name": "aws.lambda",

And traces are not connecting with logs - https://docs.datadoghq.com/tracing/connect_logs_and_traces/ - because Datadog is making query like @lambda.request_id:(fe757c85-ef6f-4c09-8e24-ca5a16edbdae) OR trace_id:8049978446405082368 (converted hex to uint64 is giving correct value). Also reading documentation https://docs.datadoghq.com/api/v1/tracing/#send-traces looks like DataDog is expecting trace_id as uint64

Even in this PR we can find

fmt.Println("Current trace ID: " + strconv.FormatUint(currentSpanContext.TraceID(), 10))

Bottom line: In current Hex format traces are not connected to Logs - it should be fixed and send as uint64

What do you think? Or maybe I am doing something wrong