Loki: support loki with streaming in dashboards (#18709)

Move some of the buffering with live streaming inside of the datasource, sending full frames instead of deltas and allow Loki in dashboards.

public/app/plugins/datasource/loki/live_streams.ts

@@ -0,0 +1,51 @@
+import { DataFrame, FieldType, parseLabels, KeyValue, CircularDataFrame } from '@grafana/data';
+import { Observable, BehaviorSubject } from 'rxjs';
+import { webSocket } from 'rxjs/webSocket';
+import { LokiResponse } from './types';
+import { finalize, map, multicast, refCount } from 'rxjs/operators';
+import { appendResponseToBufferedData } from './result_transformer';
+
+/**
+ * Maps directly to a query in the UI (refId is key)
+ */
+export interface LiveTarget {
+  query: string;
+  regexp: string;
+  url: string;
+  refId: string;
+  size: number;
+}
+
+/**
+ * Cache of websocket streams that can be returned as observable. In case there already is a stream for particular
+ * target it is returned and on subscription returns the latest dataFrame.
+ */
+export class LiveStreams {
+  private streams: KeyValue<Observable<DataFrame[]>> = {};
+
+  getStream(target: LiveTarget): Observable<DataFrame[]> {
+    let stream = this.streams[target.url];
+    if (!stream) {
+      const data = new CircularDataFrame({ capacity: target.size });
+      data.labels = parseLabels(target.query);
+      data.addField({ name: 'ts', type: FieldType.time, config: { title: 'Time' } });
+      data.addField({ name: 'line', type: FieldType.string });
+      data.addField({ name: 'labels', type: FieldType.other });
+
+      const subject = new BehaviorSubject<DataFrame[]>([]);
+      stream = webSocket(target.url).pipe(
+        finalize(() => {
+          delete this.streams[target.url];
+        }),
+        map((response: LokiResponse) => {
+          appendResponseToBufferedData(response, data);
+          return [data];
+        }),
+        multicast(subject),
+        refCount()
+      );
+      this.streams[target.url] = stream;
+    }
+    return stream;
+  }
+}