Forward oauth tokens after prometheus datasource migration (#43686)

* create the prom client

* implement lru cache of prometheus clients based on auth headers

* linter

pkg/tsdb/prometheus/promclient/cache.go

@@ -0,0 +1,55 @@
+package promclient
+
+import (
+	lru "github.com/hashicorp/golang-lru"
+	apiv1 "github.com/prometheus/client_golang/api/prometheus/v1"
+)
+
+const (
+	noPassThrough = "no-pass-through"
+)
+
+type ProviderCache struct {
+	provider promClientProvider
+	cache    *lru.Cache
+	jsonData JsonData
+}
+
+type promClientProvider interface {
+	GetClient(map[string]string) (apiv1.API, error)
+}
+
+func NewProviderCache(p promClientProvider, jd JsonData) (*ProviderCache, error) {
+	cache, err := lru.New(500)
+	if err != nil {
+		return nil, err
+	}
+
+	return &ProviderCache{
+		provider: p,
+		cache:    cache,
+		jsonData: jd,
+	}, nil
+}
+
+func (c *ProviderCache) GetClient(headers map[string]string) (apiv1.API, error) {
+	key := c.key(headers)
+	if client, ok := c.cache.Get(key); ok {
+		return client.(apiv1.API), nil
+	}
+
+	client, err := c.provider.GetClient(headers)
+	if err != nil {
+		return nil, err
+	}
+
+	c.cache.Add(key, client)
+	return client, nil
+}
+
+func (c *ProviderCache) key(headers map[string]string) string {
+	if c.jsonData.OauthPassThru {
+		return headers[authHeader] + headers[idTokenHeader]
+	}
+	return noPassThrough
+}