QAN explain and tables tabs not working after removing MySQL metrics agent

If we use only query analytics, by disabling MySQL metrics agent, the Explain and Tables tabs stop working, with the following message:

Couldn't resolve dsn, as there should be only one exporter 

See screenshots attached, for before and after.

Steps to reproduce:

1. Add all services
   1. pmm-admin add mysql ...
2. give it some time for it to collect queries correctly
3. test that Explain and Tables tabs are working correctly in QAN dashboard
4. Remove MySQL metrics exporter/agent
   1. pmm-admin list
   2. get agent ID for mysql metrics
   3. pmm-admin inventory remove agent /agent_id/68dd87c1-4246-43ef-bb6b-5f524c1089d5
5. test Explain and Tables tabs again, to find errors as shown in attached screenshots

Suggested fix:

Now we use mysqld_exporter agent as single source of credentials, I suggest to use qan agent as primary source and then if it doesn't exist check other agent types related to current service type.