Explore: Fix context view in logs, where some rows may have been filtered out. (#21729)

* Fix timestamp formats and use uid to filter context rows

* Remove timestamps from tests

public/app/core/logs_model.test.ts

@@ -223,7 +223,6 @@ describe('dataFrameToLogsModel', () => {
     expect(logsModel.rows).toHaveLength(2);
     expect(logsModel.rows).toMatchObject([
       {
-        timestamp: '2019-04-26T09:28:11.352440161Z',
         entry: 't=2019-04-26T11:05:28+0200 lvl=info msg="Initializing DatasourceCacheService" logger=server',
         labels: { filename: '/var/log/grafana/grafana.log', job: 'grafana' },
         logLevel: 'info',
@@ -231,7 +230,6 @@ describe('dataFrameToLogsModel', () => {
         uid: 'foo',
       },
       {
-        timestamp: '2019-04-26T14:42:50.991981292Z',
         entry: 't=2019-04-26T16:42:50+0200 lvl=eror msg="new token…t unhashed token=56d9fdc5c8b7400bd51b060eea8ca9d7',
         labels: { filename: '/var/log/grafana/grafana.log', job: 'grafana' },
         logLevel: 'error',