[DPE-5200] add leader info to relation data

[reneradoi]

Issue

With #95 the Discourse team requests to have the Redis Sentinel leader information in the relation data.

Solution

On updates to the Sentinel leader and when a new relation to redis is created, add the leader information to the relation data of all related applications.

Note

It also fixes an issue with metadata.yaml, which caused builds to fail: With a recent charmcraft update, the summary in metadata.yaml must not be longer than 200 chars anymore.

[alithethird]

Hi Rene! Thanks for the changes. Do you plan to also update the library to get the master ip directly? I think also the url function in the library should be updated too.

[Mehdi-Bendriss]

In general, we will rewrite the external relations lib / contract to align with the DP interfaces - as those are legacy interfaces, so I see no harm in this PR for the time being.