msdistribution_agents and msdistribution_history tables are empty, because I think these tables are used for the transactional replication only.
You are right, left join to MSmerge_agents is needed, but I am not sure about the session_id. I think the right key is the error_id
Let's compare MSmerge_history with error>0 and MSrepl_errors tables.
SELECT * FROM MSmerge_history WHERE error_id>0 ORDER BY time desc
SELECT * FROM MSrepl_errors ORDER BY time desc
MSmerge_history table has unique error_id, table MSrepl_errors not. It looks like the history information 1st comes to the MSmerge_history table, see the time columns in both tables and then more descriptive data to the MSrepl_errors table. Maybe it's even better to concatenate the string with error codes and texts from MSrepl_errors by error_id and use it for automation analysis.
I am personally not using the history table directly, because of the history retention policy. I have created a table in the local DBA_DB database and scheduled the job to run every 5 minutes. My monitoring system reads this table and sends customized alerts with the status in (5,6) or IsError >0. I can also keep the replication history much longer.
There is a query which I am using for automation:
SET nocount ON;
exec distribution.dbo.sp_replmonitorrefreshjob @iterations=1;
SELECT MSreplication_monitordata.Publisher ,
CASE WHEN MSreplication_monitordata.agent_type = 4 THEN 'Merge Agent'
END [agent_type] ,
ISNULL(( SELECT Name
WHERE server_id = MSmerge_agents.Subscriber_ID
), '') [Subscriber] ,
CASE WHEN status IN ( 5, 6 )
OR comments = 'Retrieving subscription information.' THEN 1
END [IsError] ,
comments [LastInformationMessage] ,
MSreplication_monitordata.last_distsync [LastSync] ,
LEFT JOIN distribution.dbo.MSmerge_agents ON MSmerge_agents.id = MSreplication_monitordata.agent_id
LEFT JOIN distribution.dbo.MSmerge_history ON MSreplication_monitordata.agent_id = MSmerge_history.agent_id
JOIN ( SELECT agent_id ,
MAX(ISNULL(timestamp, 0)) timestamp
GROUP BY agent_id
) a ON MSmerge_history.agent_id = a.agent_id
AND a.timestamp = MSmerge_history.timestamp
WHERE MSreplication_monitordata.agent_type IN ( 4 )