Plugins: Pass through dashboard/contextual HTTP headers to plugins/datasources (#60301)

`X-Dashboard-Uid`, `X-Datasource-Uid`, `X-Grafana-Org-Id`, `X-Panel-Id` are very useful headers set
by Grafana front-end that we would like to see on the data source as
well. This is so that it would be possible to pinpoint from where slow
queries are coming in Mimir/Thanos/Cortex/etc., for example. Relevant
Mimir code lines:
0a94f26203/pkg/frontend/transport/handler.go (L182-L184)

Tested manually that with these changes the headers are visible.

pkg/services/pluginsintegration/clientmiddleware/tracing_header_middleware.go

@@ -0,0 +1,83 @@
+package clientmiddleware
+
+import (
+	"context"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+	"github.com/grafana/grafana/pkg/plugins"
+	"github.com/grafana/grafana/pkg/services/contexthandler"
+	"github.com/grafana/grafana/pkg/services/query"
+)
+
+// NewTracingHeaderMiddleware creates a new plugins.ClientMiddleware that will
+// populate useful tracing headers on outgoing plugins.Client and HTTP
+// requests.
+// Tracing headers are X-Datasource-Uid, X-Dashboard-Uid,
+// X-Panel-Id, X-Grafana-Org-Id.
+func NewTracingHeaderMiddleware() plugins.ClientMiddleware {
+	return plugins.ClientMiddlewareFunc(func(next plugins.Client) plugins.Client {
+		return &TracingHeaderMiddleware{
+			next: next,
+		}
+	})
+}
+
+type TracingHeaderMiddleware struct {
+	next plugins.Client
+}
+
+func (m *TracingHeaderMiddleware) applyHeaders(ctx context.Context, req backend.ForwardHTTPHeaders) {
+	reqCtx := contexthandler.FromContext(ctx)
+	// If no HTTP request context then skip middleware.
+	if req == nil || reqCtx == nil || reqCtx.Req == nil {
+		return
+	}
+
+	var headersList = []string{query.HeaderPanelID, query.HeaderDashboardUID, query.HeaderDatasourceUID, `X-Grafana-Org-Id`}
+
+	for _, headerName := range headersList {
+		gotVal := reqCtx.Req.Header.Get(headerName)
+		if gotVal == "" {
+			continue
+		}
+		req.SetHTTPHeader(headerName, gotVal)
+	}
+}
+
+func (m *TracingHeaderMiddleware) QueryData(ctx context.Context, req *backend.QueryDataRequest) (*backend.QueryDataResponse, error) {
+	if req == nil {
+		return m.next.QueryData(ctx, req)
+	}
+
+	m.applyHeaders(ctx, req)
+	return m.next.QueryData(ctx, req)
+}
+
+func (m *TracingHeaderMiddleware) CallResource(ctx context.Context, req *backend.CallResourceRequest, sender backend.CallResourceResponseSender) error {
+	return m.next.CallResource(ctx, req, sender)
+}
+
+func (m *TracingHeaderMiddleware) CheckHealth(ctx context.Context, req *backend.CheckHealthRequest) (*backend.CheckHealthResult, error) {
+	if req == nil {
+		return m.next.CheckHealth(ctx, req)
+	}
+
+	m.applyHeaders(ctx, req)
+	return m.next.CheckHealth(ctx, req)
+}
+
+func (m *TracingHeaderMiddleware) CollectMetrics(ctx context.Context, req *backend.CollectMetricsRequest) (*backend.CollectMetricsResult, error) {
+	return m.next.CollectMetrics(ctx, req)
+}
+
+func (m *TracingHeaderMiddleware) SubscribeStream(ctx context.Context, req *backend.SubscribeStreamRequest) (*backend.SubscribeStreamResponse, error) {
+	return m.next.SubscribeStream(ctx, req)
+}
+
+func (m *TracingHeaderMiddleware) PublishStream(ctx context.Context, req *backend.PublishStreamRequest) (*backend.PublishStreamResponse, error) {
+	return m.next.PublishStream(ctx, req)
+}
+
+func (m *TracingHeaderMiddleware) RunStream(ctx context.Context, req *backend.RunStreamRequest, sender *backend.StreamSender) error {
+	return m.next.RunStream(ctx, req, sender)
+}