message: Add proxy endpoint for Prometheus targets
  The Thanos querier recently added support for retrieving information
  on targets from federated Prometheus instances.  This adds the new
  endpoint to the Promethus proxy.  It's not currently possible to query
  this in a tenant aware fashion, so there's no need to add it to that
  proxy.
diff: @@ -298,6 +298,7 @@ func (s *Server) HTTPHandler() http.Handler {
  rulesSourcePath = prometheusProxyEndpoint + "/api/v1/rules"
  querySourcePath = prometheusProxyEndpoint + "/api/v1/query"
  queryRangeSourcePath = prometheusProxyEndpoint + "/api/v1/query_range"
  + targetsSourcePath = prometheusProxyEndpoint + "/api/v1/targets"
  targetAPIPath = prometheusProxyEndpoint + "/api/"
  tenancyQuerySourcePath = prometheusTenancyProxyEndpoint + "/api/v1/query"
  @@ -332,6 +333,13 @@ func (s *Server) HTTPHandler() http.Handler {
  thanosProxy.ServeHTTP(w, r)
  })),
  )
  + handle(targetsSourcePath, http.StripPrefix(
  + proxy.SingleJoiningSlash(s.BaseURL.Path, targetAPIPath),
  + authHandlerWithUser(func(user *auth.User, w http.ResponseWriter, r *http.Request) {
  + r.Header.Set("Authorization", fmt.Sprintf("Bearer %s", user.Token))
  + thanosProxy.ServeHTTP(w, r)
  + })),
  + )
  // alerting (rules) are being proxied via thanos querier
  // such that both in-cluster and user workload alerts appear in console.
  