The end user's screen can be controlled if you have installed the Helpdesk agent (part of the Horizon View Agent). Just remember this is a "Enterprise" feature, so it's not included in the standard or advanced license.
Other things you can see form the Horizon Console with the heldpesk agent installed, are running process and applications for a user's session and logging related to Horizon stuff (it that's what you mean by "logs for the application/desktop activity")
If you don't have the helpdesk agent (or license), you can still see logging from the Horizon Console.
Remote control of the user sessions is still possible, if you setup the necessary GPO settings for shadowing sessions. For single-user VDI sessions you could simply use the command:
mstsc.exe /V:<vdi-computer-name> /shadow:1
Which will ask confirmation of the user to let you seem his/her screen.
In multi-user (RDSH) setup, you'll need to find out the session id of the user you want to shadow. This you can find on the server the user's logged on using e.g. task manager.
Is there a feature wherein user will not get notified when admin is viewing the screen. like End user screen monitoring ? .... just wondering if there's kind of feature horizon has ....
Be sure to check local legislation about viewing an end-users' screen without notifying the user.
That being said, not from within Horizon I think. Coming back to the command I specified before, if the correct GPO's are set to allow shadowing without notifying the user, you can change the command to:
mstsc.exe /V:<vdi-computer-name> /Shadow:1 /NoConsentPrompt