Prometheus: Improve response parser performance (#101517)

* introduce tests

* improve allocation by %61

* ~%27 improvement

goos: darwin
goarch: arm64
pkg: github.com/grafana/grafana/pkg/promlib/converter
cpu: Apple M1 Pro
                                                            │  pmem.0.txt  │             pmem.2.txt             │
                                                            │    sec/op    │   sec/op     vs base               │
ReadPrometheusStyleResult_FromFile/prom-query-range.json-10   117.72µ ± 6%   82.90µ ± 4%  -29.57% (p=0.002 n=6)

                                                            │  pmem.0.txt   │             pmem.2.txt              │
                                                            │     B/op      │     B/op      vs base               │
ReadPrometheusStyleResult_FromFile/prom-query-range.json-10   104.34Ki ± 0%   76.09Ki ± 0%  -27.08% (p=0.002 n=6)

                                                            │ pmem.0.txt  │            pmem.2.txt             │
                                                            │  allocs/op  │ allocs/op   vs base               │
ReadPrometheusStyleResult_FromFile/prom-query-range.json-10   2463.0 ± 0%   899.0 ± 0%  -63.50% (p=0.002 n=6)

* add more tests

* remove comment lines

* read string as slice to prevent type conversion

* golang lint

* revert appendValueFromString improvement

* fix merging issues

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