Loki: Create Variable Query Editor for Loki. (#54102)

* feat(loki-query-editor): create base editor component

* feat(loki-query-editor): update editor to use loki query type

* feat(loki-query-editor): add custom variable support to datasource

* feat(loki-query-editor): prevent errors when no label is present

* Add unit test for LokiMetricFindQuery

* Update datasource test

* Add component test

* Add variable query migration support

* feat(loki-query-editor): add migration support for older-format variables

* Fix enum capitalization for consistency

* Move attribute to the top of the class

* Remove unnecessary from()

* Update capitalization of new enum

* Fix enum capitalization in component

* feat(loki-query-editor): replace unnecessary class with class method

public/app/plugins/datasource/loki/migrations/variableQueryMigrations.test.ts

@@ -0,0 +1,48 @@
+import { LokiVariableQuery, LokiVariableQueryType } from '../types';
+
+import { migrateVariableQuery } from './variableQueryMigrations';
+
+describe('Loki migrateVariableQuery()', () => {
+  it('Does not migrate LokiVariableQuery instances', () => {
+    const query: LokiVariableQuery = {
+      refId: 'test',
+      type: LokiVariableQueryType.LabelValues,
+      label: 'label',
+      stream: 'stream',
+    };
+
+    expect(migrateVariableQuery(query)).toBe(query);
+    expect(migrateVariableQuery(query)).toStrictEqual(query);
+  });
+
+  it('Migrates label_names() queries', () => {
+    const query = 'label_names()';
+
+    expect(migrateVariableQuery(query)).toStrictEqual({
+      refId: 'LokiVariableQueryEditor-VariableQuery',
+      type: LokiVariableQueryType.LabelNames,
+    });
+  });
+
+  it('Migrates label_values(label) queries', () => {
+    const query = 'label_values(label)';
+
+    expect(migrateVariableQuery(query)).toStrictEqual({
+      refId: 'LokiVariableQueryEditor-VariableQuery',
+      type: LokiVariableQueryType.LabelValues,
+      label: 'label',
+      stream: undefined,
+    });
+  });
+
+  it('Migrates label_values(log stream selector, label) queries', () => {
+    const query = 'label_values(log stream selector, label)';
+
+    expect(migrateVariableQuery(query)).toStrictEqual({
+      refId: 'LokiVariableQueryEditor-VariableQuery',
+      type: LokiVariableQueryType.LabelValues,
+      label: 'label',
+      stream: 'log stream selector',
+    });
+  });
+});