Slow queries dashboard: bug fixes and a new panel (#7194)

* Slow queries dashboard: fix parsing errors on some log lines

Signed-off-by: Dimitar Dimitrov <dimitar.dimitrov@grafana.com>

* Add support for a loki datasource in `make mixin-serve`

Signed-off-by: Dimitar Dimitrov <dimitar.dimitrov@grafana.com>

* Make `query_wall_time_seconds` sortable and not hidden

This was erroneously removed

Signed-off-by: Dimitar Dimitrov <dimitar.dimitrov@grafana.com>

* Add panel with query wall time

Signed-off-by: Dimitar Dimitrov <dimitar.dimitrov@grafana.com>

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Signed-off-by: Dimitar Dimitrov <dimitar.dimitrov@grafana.com>