Queries running the longest?

Good day everyone.

Firstly I would like to thank everyone in advance for all
the help over the last couple of weeks on this mailing list as it has
helped me out a lot in my efforts to try and understand Postgresql
better.

I am however once again here with some more questions which I
hope someone can assist me with as I can not seem to find good
documentation on this as yet...

As part of our daily health checks and efforts to assist our
client to improve their performance on their postgresql databases, we
need to look at the queries that takes the longest to run and then use
explain or other methods to try and improve the queries, add indexes
where required, etc...

However, I am trying to find out how I will be able to
track / see these queries in order for me to be able to investigate
these.

the client have a total of 100 databases and to manually
monitor these will be too time consuming considering that we have many
clients to look after each day.

Can anyone perhaps point me in the right direction here, even if
it is in the form of documentation that will assist me in this.

Regards
Machiel