Query-frontend: add traceID to slow query detected log line

[abaguas]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Add traceID to slow query detected log line. The traceID is extracted from the X-Thanos-Trace-Id header, which is send in the request.
If there is no trace id the value will be empty. This behaviour is similar to the already existing log fields grafana_dashboard_uid and grafana_panel_id.

This will greatly facilitate debugging as we are often interested in analyzing the traces of the slowest queries

Verification

Built the binary, deployed it as query frontend and checked its logs:

level=info ts=2023-02-21T11:15:28.920622293Z caller=handler.go:194 org_id=anonymous msg="slow query detected" method=POST host=thanos-query-frontend-grafana.thanos.svc:9090 path=/api/v1/query time_taken=192.624733ms grafana_dashboard_uid=- grafana_panel_id=- trace_id=c33ad57d9ed443895223d5fa01499ead param_query="avg(kube_persistentvolume_capacity_bytes) by (cluster_id, persistentvolume)[1h:1m]" param_time=1676980800

[fpetkovski]

Love it, just one small nit!

[fpetkovski]

LGTM!