backend: improve parse function performance in gtime package

[itsmylife]

What this PR does / why we need it:

parse function is used in ParseDuration and ParseInterval. Those methods is used in many places in grafana/grafana. ParseDuration is especially used in settings.go to parse the config. Improving their performance will result a better start time and less memory usage.

This is also a complimentary PR for #1237

This method is using when we get the request and parse the request data into a model. Its overall impact to memory consumption is small comparing to the other parts. But considering that it's been used in many places total impact is still important.

Also I improved it in all areas that means, less allocations, less gc overheat, faster calculations.

Here is the benchmark comparison:

goos: darwin
goarch: arm64
pkg: github.com/grafana/grafana-plugin-sdk-go/backend/gtime
cpu: Apple M1 Pro
                     │  pmem.0.txt   │             pmem.2.txt             │
                     │    sec/op     │   sec/op     vs base               │
Parse/PureNumber-10    164.50n ± 11%   91.89n ± 1%  -44.14% (p=0.002 n=6)
Parse/SimpleUnit-10     79.85n ±  0%   15.21n ± 0%  -80.94% (p=0.002 n=6)
Parse/ComplexUnit-10   105.80n ±  0%   42.40n ± 1%  -59.93% (p=0.002 n=6)
Parse/DateUnit-10      148.55n ±  1%   13.27n ± 4%  -91.07% (p=0.002 n=6)
Parse/MonthUnit-10     151.40n ±  1%   13.66n ± 1%  -90.97% (p=0.002 n=6)
Parse/YearUnit-10      151.20n ±  1%   13.59n ± 0%  -91.01% (p=0.002 n=6)
geomean                 129.5n         22.95n       -82.28%

                     │  pmem.0.txt  │               pmem.2.txt               │
                     │     B/op     │    B/op     vs base                    │
Parse/PureNumber-10      80.00 ± 0%   72.00 ± 0%   -10.00% (p=0.002 n=6)
Parse/SimpleUnit-10      8.000 ± 0%   0.000 ± 0%  -100.00% (p=0.002 n=6)
Parse/ComplexUnit-10     8.000 ± 0%   0.000 ± 0%  -100.00% (p=0.002 n=6)
Parse/DateUnit-10      152.000 ± 0%   4.000 ± 0%   -97.37% (p=0.002 n=6)
Parse/MonthUnit-10     152.000 ± 0%   4.000 ± 0%   -97.37% (p=0.002 n=6)
Parse/YearUnit-10      152.000 ± 0%   4.000 ± 0%   -97.37% (p=0.002 n=6)
geomean                  51.18                    ?                      ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

                     │ pmem.0.txt │               pmem.2.txt               │
                     │ allocs/op  │ allocs/op   vs base                    │
Parse/PureNumber-10    5.000 ± 0%   4.000 ± 0%   -20.00% (p=0.002 n=6)
Parse/SimpleUnit-10    1.000 ± 0%   0.000 ± 0%  -100.00% (p=0.002 n=6)
Parse/ComplexUnit-10   1.000 ± 0%   0.000 ± 0%  -100.00% (p=0.002 n=6)
Parse/DateUnit-10      3.000 ± 0%   1.000 ± 0%   -66.67% (p=0.002 n=6)
Parse/MonthUnit-10     3.000 ± 0%   1.000 ± 0%   -66.67% (p=0.002 n=6)
Parse/YearUnit-10      3.000 ± 0%   1.000 ± 0%   -66.67% (p=0.002 n=6)
geomean                2.265                    ?                      ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

[beejeebus]

LGTM, one nit in the parse function.

[beejeebus]

I think this should be:

  if inp == "" {
    return 0, "", errors.New("empty input")
  }

then the following code can come out of the if block.