Fix panic at startup when Mimir is running in monolithic mode and query sharding is enabled

[pracucci]

What this PR does

I'm doing some tests and I've just realized that Mimir panics at startup when running in monolithic mode and query sharding is enabled, because of conflicting metrics registered by PromQL engine:

panic: a previously registered descriptor with the same fully-qualified name as Desc{fqName: "prometheus_engine_queries", help: "The current number of queries being executed or waiting.", constLabels: {engine="querier"}, variableLabels: []} has different label names or a different help string

goroutine 1 [running]:
github.com/prometheus/client_golang/prometheus.(*wrappingRegisterer).MustRegister(0xc010ee4f90, {0xc01102c000, 0x5, 0x6})
	/go/src/github.com/grafana/mimir/vendor/github.com/prometheus/client_golang/prometheus/wrap.go:104 +0x151
github.com/prometheus/prometheus/promql.NewEngine({{0x235b9a0, 0xc00036aaa0}, {0x237bed0, 0xc010ee4f90}, 0x2faf080, 0x45d964b800, {0x237b510, 0xc010ec0f00}, 0x45d964b800, 0xc0005d37d0, ...})
	/go/src/github.com/grafana/mimir/vendor/github.com/prometheus/prometheus/promql/engine.go:350 +0x8f0
github.com/grafana/mimir/pkg/querier.New({0x0, 0x1, 0x2a9079602000, 0x274a48a78000, 0x8bb2c97000, {0x0, {{0x0, 0x0}, {0x0, 0x0}, ...}}, ...}, ...)
	/go/src/github.com/grafana/mimir/pkg/querier/querier.go:134 +0x3a5
github.com/grafana/mimir/pkg/mimir.(*Mimir).initQueryable(0xc00062e000)
	/go/src/github.com/grafana/mimir/pkg/mimir/modules.go:311 +0x1c5
github.com/grafana/dskit/modules.(*Manager).initModule(0xc00000ce88, {0x7ffef8c6a608, 0x1e16cab}, 0x1e, 0x0)
	/go/src/github.com/grafana/mimir/vendor/github.com/grafana/dskit/modules/modules.go:120 +0x224
github.com/grafana/dskit/modules.(*Manager).InitModuleServices(0x1aee9c0, {0xc00093e960, 0x1, 0x4})
	/go/src/github.com/grafana/mimir/vendor/github.com/grafana/dskit/modules/modules.go:92 +0x10c
github.com/grafana/mimir/pkg/mimir.(*Mimir).Run(0xc00062e000)
	/go/src/github.com/grafana/mimir/pkg/mimir/mimir.go:438 +0x21c
main.main()
	/go/src/github.com/grafana/mimir/cmd/mimir/main.go:204 +0xa90

This PR fixes it, adding engine label to PromQL engine registerer used by query-frontend. The engine label is already injected by querier and ruler, so in this PR I'm just following what we already did there.

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]