Prometheus: Fix term completion that contain keywords (#21295)

- `unless`, `or` and `and` are binary operators
- if they appear in a query the query was broken up so the suggestor
only works on the current term
- this change fixes the splitter logic to make sure those keywords are
not inside a word

public/app/plugins/datasource/prometheus/language_provider.test.ts

@@ -49,6 +49,7 @@ describe('Language completion provider', () => {
       expect(cleanText('foo < bar')).toBe('bar');
       expect(cleanText('foo >= bar')).toBe('bar');
       expect(cleanText('foo <= bar')).toBe('bar');
+      expect(cleanText('memory')).toBe('memory');
     });
 
     it('removes aggregation syntax', () => {