feat: use pools for attributes and options

[costela]

This PR includes 2 minor and one sensible performance improvements:

• minor: avoid the memory cost of a string context key
• minor: avoid nil-checks by directly setting spanNameFunc
• use slice pools for attributes and options

The last one in particular has a sensible impact on allocations:

cpu: 12th Gen Intel(R) Core(TM) i7-1250U
          │  old.bench  │           new.bench           │
          │   sec/op    │   sec/op     vs base          │
Tracer-12   1.179m ± 3%   1.174m ± 3%  ~ (p=0.684 n=10)

          │   old.bench   │              new.bench               │
          │     B/op      │     B/op      vs base                │
Tracer-12   10.261Ki ± 0%   9.221Ki ± 0%  -10.13% (p=0.000 n=10)

          │ old.bench  │             new.bench              │
          │ allocs/op  │ allocs/op   vs base                │
Tracer-12   93.00 ± 0%   76.00 ± 0%  -18.28% (p=0.000 n=10)