Derive series from pprof sample labels

[kolesnikovae]

Currently, distributor does not preserve sample labels at normalisation. For example, samples

stack trace A, {foo=bar,baz=quz}, 1
stack trace A, {foo=fred,baz=zoo}, 1

cut to:

stack trace A, {foo=bar,baz=quz}, 2

---

With this change, labeled pprof samples are exported as indpendent series. Thus, if a profile with external label set

{service_name="my-service", instance="x"}

contains samples labeled with span_name="A" and span_name="B", distributor will extract them and send to ingesters in accordance to the newly created series labels:

{service_name="my-service", instance="x"} // Contains no samples with labels
{service_name="my-service", instance="x", span_name="A"} // Only contains samples with the span_name="A" label
{service_name="my-service", instance="x", span_name="B"} // Only contains samples with the span_name="B" label

In practice this allows for more granular queries. For example, profile of an HTTP/gRPC endpoint:

Screen.Recording.2023-09-21.at.18.10.25.mov

[cyriltovena]

LGTM

[korniltsev]

nit: maybe add go.cpu.labels.pprof to pkg/test/integration/ingest_pprof_test.go

[kolesnikovae]

Should also fix #2429