Only convert to any on KindAny values

[database64128]

Indiscriminately calling slog.Value.Any() on the value causes extra allocations for non-any values. Benchmark shows that for each string attribute, an extra 16-byte allocation is incurred.

This commit adds a check so Any() is only called on values of KindAny.

[lmittmann]

Thanks for the PR!

                                      │     old      │                new                  │
                                      │    sec/op    │   sec/op     vs base                │
LogAttrs/h=tint/5_args-8                 1.610µ ± 1%   1.458µ ± 1%   -9.44% (p=0.000 n=10)
LogAttrs/h=tint/5_args_custom_level-8    1.613µ ± 0%   1.479µ ± 1%   -8.31% (p=0.000 n=10)
LogAttrs/h=tint/10_args-8                2.872µ ± 0%   2.605µ ± 1%   -9.31% (p=0.000 n=10)
LogAttrs/h=tint/40_args-8               10.164µ ± 1%   9.087µ ± 1%  -10.60% (p=0.000 n=10)
geomean                                  2.950µ        2.673µ        -9.42%

                                      │     old      │                 new                  │
                                      │     B/op     │     B/op      vs base                │
LogAttrs/h=tint/5_args-8                  96.00 ± 0%     36.00 ± 0%  -62.50% (p=0.000 n=10)
LogAttrs/h=tint/5_args_custom_level-8     96.00 ± 0%     36.00 ± 0%  -62.50% (p=0.000 n=10)
LogAttrs/h=tint/10_args-8                 400.0 ± 0%     280.0 ± 0%  -30.00% (p=0.000 n=10)
LogAttrs/h=tint/40_args-8               2.126Ki ± 0%   1.657Ki ± 0%  -22.05% (p=0.000 n=10)
geomean                                   299.3          157.5       -47.37%

                                      │    old      │                new                 │
                                      │  allocs/op  │ allocs/op   vs base                │
LogAttrs/h=tint/5_args-8                 6.000 ± 0%   2.000 ± 0%  -66.67% (p=0.000 n=10)
LogAttrs/h=tint/5_args_custom_level-8    6.000 ± 0%   2.000 ± 0%  -66.67% (p=0.000 n=10)
LogAttrs/h=tint/10_args-8               13.000 ± 0%   5.000 ± 0%  -61.54% (p=0.000 n=10)
LogAttrs/h=tint/40_args-8                49.00 ± 0%   17.00 ± 0%  -65.31% (p=0.000 n=10)
geomean                                  12.31        4.294       -65.11%