benchstat:
```
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/trace
cpu: 11th Gen Intel(R) Core(TM) i5-11400H @ 2.70GHz
                                 │   new.txt    │              new1.txt               │
                                 │    sec/op    │   sec/op     vs base                │
TraceStart/with_a_simple_span-12   451.0n ±  5%   375.0n ± 1%  -16.85% (p=0.000 n=10)
TraceStart/with_several_links-12   595.8n ±  3%   501.7n ± 1%  -15.80% (p=0.000 n=10)
TraceStart/with_attributes-12      644.5n ± 10%   569.5n ± 4%  -11.63% (p=0.000 n=10)
geomean                            557.4n         474.9n       -14.79%

                                 │  new.txt   │              new1.txt               │
                                 │    B/op    │    B/op     vs base                 │
TraceStart/with_a_simple_span-12   496.0 ± 0%   496.0 ± 0%       ~ (p=1.000 n=10) ¹
TraceStart/with_several_links-12   672.0 ± 0%   672.0 ± 0%       ~ (p=1.000 n=10) ¹
TraceStart/with_attributes-12      752.0 ± 0%   752.0 ± 0%       ~ (p=1.000 n=10) ¹
geomean                            630.5        630.5       +0.00%
¹ all samples are equal

                                 │  new.txt   │              new1.txt               │
                                 │ allocs/op  │ allocs/op   vs base                 │
TraceStart/with_a_simple_span-12   2.000 ± 0%   2.000 ± 0%       ~ (p=1.000 n=10) ¹
TraceStart/with_several_links-12   3.000 ± 0%   3.000 ± 0%       ~ (p=1.000 n=10) ¹
TraceStart/with_attributes-12      4.000 ± 0%   4.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                            2.884        2.884       +0.00%
¹ all samples are equal
```

Grow increases the slice's capacity, if necessary, to guarantee space for another n elements.
This change now ensures that `n` is the amount of elements to grow and not the capacity amount.

Co-authored-by: Damien Mathieu <[email protected]>