specialize simplelru(span's lruMap) for attributes only

[paivagustavo]

This specialize the simplelru map for attributes only, reducing the number of usages of interface{}, consequently reducing the number of allocations.

The reduction of the number of allocs range from 8%~42% on our current benchmarks. Related benchmarks:

name                                             old time/op    new time/op    delta
StartEndSpan/AlwaysSample-6                        2.47µs ±27%    2.17µs ±28%  -11.93%  (p=0.004 n=20+20)
SpanWithAttributes_4/AlwaysSample-6                4.70µs ±30%    4.43µs ±17%     ~     (p=0.515 n=20+18)
Span_SetAttributes/SetAttribute/AlwaysSample-6     4.21µs ±37%    4.62µs ±12%   +9.82%  (p=0.018 n=20+20)
Span_SetAttributes/SetAttributes/AlwaysSample-6    5.44µs ±29%    4.17µs ±28%  -23.42%  (p=0.000 n=20+20)
SpanWithAttributes_8/AlwaysSample-6                8.09µs ±15%    6.00µs ±27%  -25.79%  (p=0.000 n=20+20)
SpanWithAttributes_all/AlwaysSample-6              9.41µs ±36%    8.62µs ± 9%     ~     (p=0.256 n=20+20)
SpanWithAttributes_all_2x/AlwaysSample-6           17.5µs ±26%    12.9µs ±31%  -26.49%  (p=0.000 n=20+20)

name                                             old alloc/op   new alloc/op   delta
StartEndSpan/AlwaysSample-6                          816B ± 0%      800B ± 0%   -1.96%  (p=0.000 n=20+20)
SpanWithAttributes_4/AlwaysSample-6                1.74kB ± 0%    1.60kB ± 0%   -8.26%  (p=0.000 n=20+20)
Span_SetAttributes/SetAttribute/AlwaysSample-6     1.55kB ± 0%    1.41kB ± 0%   -9.28%  (p=0.000 n=20+20)
Span_SetAttributes/SetAttributes/AlwaysSample-6    1.74kB ± 0%    1.60kB ± 0%   -8.26%  (p=0.000 n=20+20)
SpanWithAttributes_8/AlwaysSample-6                2.45kB ± 0%    2.18kB ± 0%  -11.11%  (p=0.000 n=20+20)
SpanWithAttributes_all/AlwaysSample-6              3.22kB ± 0%    2.88kB ± 0%  -10.43%  (p=0.000 n=20+20)
SpanWithAttributes_all_2x/AlwaysSample-6           6.04kB ± 0%    5.38kB ± 0%  -10.86%  (p=0.000 n=14+18)

name                                             old allocs/op  new allocs/op  delta
StartEndSpan/AlwaysSample-6                          12.0 ± 0%      11.0 ± 0%   -8.33%  (p=0.000 n=20+20)
SpanWithAttributes_4/AlwaysSample-6                  30.0 ± 0%      21.0 ± 0%  -30.00%  (p=0.000 n=20+20)
Span_SetAttributes/SetAttribute/AlwaysSample-6       29.0 ± 0%      20.0 ± 0%  -31.03%  (p=0.000 n=20+20)
Span_SetAttributes/SetAttributes/AlwaysSample-6      33.0 ± 0%      24.0 ± 0%  -27.27%  (p=0.000 n=20+20)
SpanWithAttributes_8/AlwaysSample-6                  46.0 ± 0%      29.0 ± 0%  -36.96%  (p=0.000 n=20+20)
SpanWithAttributes_all/AlwaysSample-6                55.0 ± 0%      34.0 ± 0%  -38.18%  (p=0.000 n=20+20)
SpanWithAttributes_all_2x/AlwaysSample-6             96.0 ± 0%      55.0 ± 0%  -42.71%  (p=0.000 n=20+20)

If this is is something we are willing to do, we should probably do the same for events and links as well.