Explore: POC for datasource query importers

Explore is about keeping context between datasources if possible. When
changing from metrics to logging, some of the filtering can be kept to
narrow down logging streams relevant to the metrics.

- adds `importQueries` function in language providers
- query import dependent on origin datasource
- implemented  prometheus-to-logging import: keeping label selectors
  that are common to both datasources
- added types

public/app/plugins/datasource/prometheus/language_utils.ts

@@ -24,8 +24,8 @@ export function processLabels(labels, withName = false) {
 }
 
 // const cleanSelectorRegexp = /\{(\w+="[^"\n]*?")(,\w+="[^"\n]*?")*\}/;
-const selectorRegexp = /\{[^}]*?\}/;
-const labelRegexp = /\b(\w+)(!?=~?)("[^"\n]*?")/g;
+export const selectorRegexp = /\{[^}]*?\}/;
+export const labelRegexp = /\b(\w+)(!?=~?)("[^"\n]*?")/g;
 export function parseSelector(query: string, cursorOffset = 1): { labelKeys: any[]; selector: string } {
   if (!query.match(selectorRegexp)) {
     // Special matcher for metrics