model: reduce allocations in Time.UnmarshalJSON

[bboreham]

The basic idea was to use strings.Cut instead of strings.Split.

However I also removed another allocation if the decimal was smaller than three figures, and fixed a bug for negative times.

Benchmarks:

goos: linux
goarch: amd64
pkg: github.com/prometheus/common/model
cpu: Intel(R) Core(TM) i7-14700K
                                │   before    │               after                │
                                │   sec/op    │   sec/op     vs base               │
UnmarshalTime/1780924784-28       39.89n ± 0%   18.98n ± 1%  -52.42% (p=0.002 n=6)
UnmarshalTime/1780924784.01-28    67.99n ± 0%   26.68n ± 1%  -60.76% (p=0.002 n=6)
UnmarshalTime/1780924784.001-28   53.84n ± 1%   27.64n ± 0%  -48.66% (p=0.002 n=6)
geomean                           52.66n        24.10n       -54.23%

                                │   before   │                 after                 │
                                │    B/op    │   B/op     vs base                    │
UnmarshalTime/1780924784-28       32.00 ± 0%   0.00 ± 0%  -100.00% (p=0.002 n=6)
UnmarshalTime/1780924784.01-28    48.00 ± 0%   0.00 ± 0%  -100.00% (p=0.002 n=6)
UnmarshalTime/1780924784.001-28   48.00 ± 0%   0.00 ± 0%  -100.00% (p=0.002 n=6)
geomean                           41.93                   ?                      ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

                                │   before   │                 after                  │
                                │ allocs/op  │ allocs/op   vs base                    │
UnmarshalTime/1780924784-28       2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.002 n=6)
UnmarshalTime/1780924784.01-28    3.000 ± 0%   0.000 ± 0%  -100.00% (p=0.002 n=6)
UnmarshalTime/1780924784.001-28   2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.002 n=6)
geomean                           2.289                    ?                      ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean