Split the span start/end benchmarks and test start with links and attributes

[dmathieu]

I was looking at the trace benchmarks, and noticed this one, which says it tests "span start", but ending the span is included within the data.
So this change removes ending the span from the computation, and adds a new benchmark which only computes span end.

benchstat for span start:

pkg: go.opentelemetry.io/otel/sdk/trace
              │ bench-main  │            bench-branch            │
              │   sec/op    │   sec/op     vs base               │
TraceStart-10   725.6n ± 3%   667.2n ± 2%  -8.04% (p=0.000 n=10)

              │ bench-main │          bench-branch          │
              │    B/op    │    B/op     vs base            │
TraceStart-10   704.0 ± 0%   704.0 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

              │ bench-main │          bench-branch          │
              │ allocs/op  │ allocs/op   vs base            │
TraceStart-10   14.00 ± 0%   14.00 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

Benchmark for span end:

BenchmarkSpanEnd-10     16486819               147.7 ns/op             0 B/op          0 allocs/op

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.4%. Comparing base (82fe9aa) to head (293e66b).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5554   +/-   ##
=====================================
  Coverage   84.4%   84.4%           
=====================================
  Files        270     270           
  Lines      22336   22336           
=====================================
+ Hits       18871   18873    +2     
+ Misses      3134    3132    -2     
  Partials     331     331           

see 1 file with indirect coverage changes

[dmathieu]

I have also extended the benchmark to include links and attributes. Current results:

pkg: go.opentelemetry.io/otel/sdk/trace
BenchmarkTraceStart/with_a_simple_span-10                1822948               671.1 ns/op           704 B/op         14 allocs/op
BenchmarkTraceStart/with_several_links-10                1500474               840.0 ns/op           880 B/op         15 allocs/op
BenchmarkTraceStart/with_attributes-10                   1349312               869.6 ns/op           960 B/op         16 allocs/op
PASS