Datasource: Shared HTTP client provider for core backend data sources and any data source using the data source proxy (#33439)

Uses new httpclient package from grafana-plugin-sdk-go introduced 
via grafana/grafana-plugin-sdk-go#328. 
Replaces the GetHTTPClient, GetTransport, GetTLSConfig methods defined 
on DataSource model.
Longer-term the goal is to migrate core HTTP backend data sources to use the 
SDK contracts and using httpclient.Provider for creating HTTP clients and such.

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/infra/httpclient/httpclientprovider/user_agent_middleware_test.go

@@ -0,0 +1,82 @@
+package httpclientprovider
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCustomHeadersMiddleware(t *testing.T) {
+	t.Run("Without user agent set should return next http.RoundTripper", func(t *testing.T) {
+		ctx := &testContext{}
+		finalRoundTripper := ctx.createRoundTripper("finalrt")
+		mw := SetUserAgentMiddleware("")
+		rt := mw.CreateMiddleware(httpclient.Options{}, finalRoundTripper)
+		require.NotNil(t, rt)
+		middlewareName, ok := mw.(httpclient.MiddlewareName)
+		require.True(t, ok)
+		require.Equal(t, SetUserAgentMiddlewareName, middlewareName.MiddlewareName())
+
+		req, err := http.NewRequest(http.MethodGet, "http://", nil)
+		require.NoError(t, err)
+		res, err := rt.RoundTrip(req)
+		require.NoError(t, err)
+		require.NotNil(t, res)
+		if res.Body != nil {
+			require.NoError(t, res.Body.Close())
+		}
+		require.Len(t, ctx.callChain, 1)
+		require.ElementsMatch(t, []string{"finalrt"}, ctx.callChain)
+	})
+
+	t.Run("With user agent set should apply HTTP headers to the request", func(t *testing.T) {
+		ctx := &testContext{}
+		finalRoundTripper := ctx.createRoundTripper("final")
+		mw := SetUserAgentMiddleware("Grafana/8.0.0")
+		rt := mw.CreateMiddleware(httpclient.Options{}, finalRoundTripper)
+		require.NotNil(t, rt)
+		middlewareName, ok := mw.(httpclient.MiddlewareName)
+		require.True(t, ok)
+		require.Equal(t, SetUserAgentMiddlewareName, middlewareName.MiddlewareName())
+
+		req, err := http.NewRequest(http.MethodGet, "http://", nil)
+		require.NoError(t, err)
+		res, err := rt.RoundTrip(req)
+		require.NoError(t, err)
+		require.NotNil(t, res)
+		if res.Body != nil {
+			require.NoError(t, res.Body.Close())
+		}
+		require.Len(t, ctx.callChain, 1)
+		require.ElementsMatch(t, []string{"final"}, ctx.callChain)
+
+		require.Equal(t, "Grafana/8.0.0", req.Header.Get("User-Agent"))
+	})
+
+	t.Run("With user agent set, but request already has User-Agent header set should not apply HTTP headers to the request", func(t *testing.T) {
+		ctx := &testContext{}
+		finalRoundTripper := ctx.createRoundTripper("final")
+		mw := SetUserAgentMiddleware("Grafana/8.0.0")
+		rt := mw.CreateMiddleware(httpclient.Options{}, finalRoundTripper)
+		require.NotNil(t, rt)
+		middlewareName, ok := mw.(httpclient.MiddlewareName)
+		require.True(t, ok)
+		require.Equal(t, SetUserAgentMiddlewareName, middlewareName.MiddlewareName())
+
+		req, err := http.NewRequest(http.MethodGet, "http://", nil)
+		require.NoError(t, err)
+		req.Header.Set("User-Agent", "ua")
+		res, err := rt.RoundTrip(req)
+		require.NoError(t, err)
+		require.NotNil(t, res)
+		if res.Body != nil {
+			require.NoError(t, res.Body.Close())
+		}
+		require.Len(t, ctx.callChain, 1)
+		require.ElementsMatch(t, []string{"final"}, ctx.callChain)
+
+		require.Equal(t, "ua", req.Header.Get("User-Agent"))
+	})
+}