mimirtool: fix panic in loadgen command

[narqo]

What this PR does

This one fixes the panic in mimirtool loadgen

› go run ./cmd/mimirtool/ loadgen --write-url='http://127.0.0.1:9009/api/v1/push'
2024/03/14 20:23:59 setting up write load gen:
  url=http://127.0.0.1:9009/api/v1/push
  parallelism: 10
  active_series: 1000
 interval: 15s
2024/03/14 20:23:59 query load generation is disabled, -query-url flag has not been set
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x100c3892c]

goroutine 95 [running]:
github.com/prometheus/client_golang/prometheus.(*HistogramVec).GetMetricWithLabelValues(0x140008ef080?, {0x1400127be30?, 0x103e3f900?, 0x140015a4000?})
	/Users/v/Documents/Code/grafana/mimir-main/vendor/github.com/prometheus/client_golang/prometheus/histogram.go:1165 +0x1c
github.com/prometheus/client_golang/prometheus.(*HistogramVec).WithLabelValues(...)
	/Users/v/Documents/Code/grafana/mimir-main/vendor/github.com/prometheus/client_golang/prometheus/histogram.go:1198
github.com/grafana/mimir/pkg/mimirtool/commands.(*LoadgenCommand).runBatch(0x14000c85110, 0x2bc, 0x320)
	/Users/v/Documents/Code/grafana/mimir-main/pkg/mimirtool/commands/loadgen.go:238 +0x4a0
github.com/grafana/mimir/pkg/mimirtool/commands.(*LoadgenCommand).runScrape(0x14000c85110, 0x2bc, 0x320)
	/Users/v/Documents/Code/grafana/mimir-main/pkg/mimirtool/commands/loadgen.go:194 +0x60
github.com/grafana/mimir/pkg/mimirtool/commands.(*LoadgenCommand).runWriteShard(0x14000c85110, 0x2bc, 0x320)
	/Users/v/Documents/Code/grafana/mimir-main/pkg/mimirtool/commands/loadgen.go:186 +0x78
created by github.com/grafana/mimir/pkg/mimirtool/commands.(*LoadgenCommand).run in goroutine 1
	/Users/v/Documents/Code/grafana/mimir-main/pkg/mimirtool/commands/loadgen.go:154 +0x698
exit status 2

The issues that command initializes local metrics in the different instance of LoadgenCommand variable, than what executes the command's action.

I guess, the part that no one's bumped into that for the past 4 years, speaks for how popular the command is 🤣

Which issue(s) this PR fixes or relates to

n/a

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.