Prometheus: Use contextual middleware for req headers and simplify client creation (#51061)

* Use contextual middleware and simplify client creation

* Fix tests

* Add test for the header propagation

* Fix tests and lint

* Update pkg/tsdb/prometheus/prometheus.go

Co-authored-by: ismail simsek <ismailsimsek09@gmail.com>

Co-authored-by: ismail simsek <ismailsimsek09@gmail.com>

pkg/tsdb/prometheus/utils/utils.go

@@ -0,0 +1,42 @@
+package utils
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+	"github.com/grafana/grafana/pkg/infra/tracing"
+	"go.opentelemetry.io/otel/attribute"
+)
+
+// GetJsonData just gets the json in easier to work with type. It's used on multiple places which isn't super effective
+// but only when creating a client which should not happen often anyway.
+func GetJsonData(settings backend.DataSourceInstanceSettings) (map[string]interface{}, error) {
+	var jsonData map[string]interface{}
+	err := json.Unmarshal(settings.JSONData, &jsonData)
+	if err != nil {
+		return nil, fmt.Errorf("error unmarshalling JSONData: %w", err)
+	}
+	return jsonData, nil
+}
+
+type Attribute struct {
+	Key   string
+	Value interface{}
+	Kv    attribute.KeyValue
+}
+
+// StartTrace setups a trace but does not panic if tracer is nil which helps with testing
+func StartTrace(ctx context.Context, tracer tracing.Tracer, name string, attributes []Attribute) (context.Context, func()) {
+	if tracer == nil {
+		return ctx, func() {}
+	}
+	ctx, span := tracer.Start(ctx, name)
+	for _, attr := range attributes {
+		span.SetAttributes(attr.Key, attr.Value, attr.Kv)
+	}
+	return ctx, func() {
+		span.End()
+	}
+}