Tempo: Interpolate variables in traceQL queries

When sending query we have to first interpolate any variables there https://github.com/grafana/grafana/blob/98053cfde815f588e1c96ed51f225ea1cf14a508/public/app/plugins/datasource/tempo/datasource.ts#L183. 

Similar to prometheus and other datasources: https://github.com/grafana/grafana/blob/e59ddd6bc5630625e49cafc965764cd8faa23cd6/public/app/plugins/datasource/prometheus/datasource.tsx#L1182