contrib/internal/httptrace: improve performance of span starts

[knusbaum]

What does this PR do?

This commit improves the performance of starting a span by reducing work
done, especially allocations.

Motivation

We were seeing a large increase in memory and CPU usage while starting spans compared to previous versions.

Describe how to test/QA your changes

Reviewer's Checklist

• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[knusbaum]

$ benchstat old.bench new.bench 
name                 old time/op    new time/op    delta
StartRequestSpan-16     788ns ±13%     160ns ± 6%  -79.64%  (p=0.000 n=10+10)

name                 old alloc/op   new alloc/op   delta
StartRequestSpan-16      768B ± 0%      224B ± 0%  -70.83%  (p=0.000 n=10+10)

name                 old allocs/op  new allocs/op  delta
StartRequestSpan-16      21.0 ± 0%       5.0 ± 0%  -76.19%  (p=0.000 n=10+10)

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-07-19 19:53:56

Comparing candidate commit c65db5c in PR branch knusbaum/http-perf with baseline commit f905f70 in branch release-v1.53.x.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 27 metrics, 0 unstable metrics.

[knusbaum]

New benchstat output:

$ benchstat old.bench new.bench 
name                 old time/op    new time/op    delta
StartRequestSpan-16    1.08µs ±13%    0.35µs ± 3%  -67.92%  (p=0.000 n=10+10)

name                 old alloc/op   new alloc/op   delta
StartRequestSpan-16      768B ± 0%      264B ± 0%  -65.62%  (p=0.000 n=10+10)

name                 old allocs/op  new allocs/op  delta
StartRequestSpan-16      21.0 ± 0%      10.0 ± 0%  -52.38%  (p=0.000 n=10+10)