Prometheus: Introduce prometheus backend library (#83952)

* Move files to prometheus-library

* refactor core prometheus to use prometheus-library

* modify client transport options

* mock

* have a type

* import aliases

* rename

* call the right method

* remove unrelated test from the library

* update codeowners

* go work sync

* update go.work.sum

* make swagger-clean && make openapi3-gen

* add promlib to makefile

* remove clilogger

* Export the function

* update unit test

* add prometheus_test.go

* fix mock type

* use mapUtil from grafana-plugin-sdk-go

pkg/promlib/querydata/exemplar/sampler.go

@@ -0,0 +1,46 @@
+package exemplar
+
+import (
+	"sort"
+	"time"
+
+	"github.com/grafana/grafana/pkg/promlib/models"
+)
+
+type Sampler interface {
+	Add(models.Exemplar)
+	SetStep(time.Duration)
+	Sample() []models.Exemplar
+	Reset()
+}
+
+var _ Sampler = (*NoOpSampler)(nil)
+
+type NoOpSampler struct {
+	exemplars []models.Exemplar
+}
+
+func NewNoOpSampler() Sampler {
+	return &NoOpSampler{
+		exemplars: []models.Exemplar{},
+	}
+}
+
+func (e *NoOpSampler) Add(ex models.Exemplar) {
+	e.exemplars = append(e.exemplars, ex)
+}
+
+func (e *NoOpSampler) SetStep(time.Duration) {
+	// noop
+}
+
+func (e *NoOpSampler) Sample() []models.Exemplar {
+	sort.SliceStable(e.exemplars, func(i, j int) bool {
+		return e.exemplars[i].Timestamp.Before(e.exemplars[j].Timestamp)
+	})
+	return e.exemplars
+}
+
+func (e *NoOpSampler) Reset() {
+	e.exemplars = []models.Exemplar{}
+}