ddtrace/mocktracer: use lock in String method

[phoenix2x]

What does this PR do?

Use RLock in the mockspan String method to fix the "data race" error when running tests with -race flag. All other methods do use lock so this seems to be an oversight.

Motivation

WARNING: DATA RACE
Write at 0x00c00039b650 by goroutine 16:
  runtime.mapassign_faststr()
      /usr/local/go/src/runtime/map_faststr.go:203 +0x0
  gopkg.in/DataDog/dd-trace-go.v1/ddtrace/mocktracer.(*mockspan).SetTag()
      /home/circleci/project/vendor/gopkg.in/DataDog/dd-trace-go.v1/ddtrace/mocktracer/mockspan.go:130 +0x304

Previous read at 0x00c00039b650 by goroutine 28:
  reflect.maplen()
      /usr/local/go/src/runtime/map.go:1394 +0x0
  reflect.Value.lenNonSlice()
      /usr/local/go/src/reflect/value.go:1630 +0x26d
  reflect.Value.Len()
      /usr/local/go/src/reflect/value.go:1619 +0x11a
  internal/fmtsort.Sort()
      /usr/local/go/src/internal/fmtsort/sort.go:58 +0x101
  fmt.(*pp).printValue()
      /usr/local/go/src/fmt/print.go:800 +0x1164
  fmt.(*pp).printArg()
      /usr/local/go/src/fmt/print.go:743 +0xe17
  fmt.(*pp).doPrintf()
      /usr/local/go/src/fmt/print.go:1057 +0x499
  fmt.Sprintf()
      /usr/local/go/src/fmt/print.go:219 +0x67
  gopkg.in/DataDog/dd-trace-go.v1/ddtrace/mocktracer.(*mockspan).String()
      /home/circleci/project/vendor/gopkg.in/DataDog/dd-trace-go.v1/ddtrace/mocktracer/mockspan.go:221 +0x22a

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[katiehockman]

Thank you!!

[katiehockman]

/merge

[dd-devflow]

🚂 MergeQueue

This merge request is not mergeable yet, because of pending checks/missing approvals. It will be added to the queue as soon as checks pass and/or get approvals. You can remove it from the waiting list with /remove command.

you can cancel this operation by commenting your pull request with /merge -c!

[dd-devflow]

🚂 MergeQueue

Added to the queue.

This build is going to start soon! (estimated merge in less than 8m)

you can cancel this operation by commenting your pull request with /merge -c!