remove useless allocation in TraceState.copyOf()

[SylvainJuge]

What does this PR do?

Fixes #1503

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix tested locally with async-profiler

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: [Pull request #1508 updated]

• Start Time: 2020-11-24T15:10:26.890+0000

• Duration: 46 min 11 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1616
Skipped	12
Total	1628

[eyalkoren]

Leaking a member of a pooled object to another pooled object is very problematic.

I think we should avoid adding the header field to the tracestate list (in TraceState#set()) and adjust the TraceState#toTextHeader() method (as well as the TraceState.TextTracestateAppender#join() method) to append it only when required.
TraceState#copyFrom() already takes care of copying the header value.
This probably means we should change tracestate to be a list of Strings, as opposed to what I previously suggested...

One additional thing to fix is resetting header in TraceState#resetState.

[codecov-io]

Codecov Report

Merging #1508 (043679b) into master (7233789) will decrease coverage by 0.00%.
The diff coverage is 94.44%.

@@ Coverage Diff @@ ## master #1508 +/- ## ============================================ - Coverage 58.96% 58.96% -0.01%  Complexity 92 92 ============================================ Files 392 392 Lines 17638 17632 -6 Branches 2447 2446 -1 ============================================ - Hits 10401 10397 -4  + Misses 6520 6517 -3  - Partials 717 718 +1 

Impacted Files	Coverage Δ	Complexity Δ
...elastic/apm/agent/impl/transaction/TraceState.java	93.68% <88.88%> (-0.44%)	0.00 <0.00> (ø)
...astic/apm/agent/impl/sampling/ConstantSampler.java	100.00% <100.00%> (ø)	0.00 <0.00> (ø)
...ic/apm/agent/impl/sampling/ProbabilitySampler.java	100.00% <100.00%> (ø)	0.00 <0.00> (ø)
...astic/apm/agent/impl/transaction/TraceContext.java	75.00% <100.00%> (ø)	0.00 <0.00> (ø)
...o/elastic/apm/agent/profiler/SamplingProfiler.java	72.72% <0.00%> (-0.24%)	0.00% <0.00%> (ø%)
...ic/apm/agent/profiler/collections/LongHashSet.java	17.06% <0.00%> (+1.19%)	0.00% <0.00%> (ø%)

---

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 7233789...043679b. Read the comment docs.

[eyalkoren]

LGTM.
Few minor comment.

[eyalkoren]

👍 Can be added to the javadoc