Displaying star rating with Discourse is easy. I am happy to share, It can be done in simple few steps
Process
Setup Google Tag Manager for your Discourse Application
Add a new HTML Tag containing JSON-LD markup
Filter trigger by page view for particular URL
Publish, wait for 24 hrs to see the result in SERPs.
Thanks
Interesting. Where are you getting the rating from, Google?
Yes, it is Google SERPs. 
Looks interesting and useful for other types of integrations with tag manager. Can you show what to add in head section of Discourse?
Oh but Discourse has a setting to do that for you. Search settings for GTM.
Good to know, we can add GTM script via Settings. Thanks for the hint about better way.
Updated, above quick guide.
@neil There is one problem when I add GTM script via Discourse settings, it places inside body.
Expected: It should be in the head.
Previously, it was recommended by Google to place in the body.
But now, they have updated their guidelines: “Paste this code as high in the <head> of the page as possible”.
Can we make this possible?
Thanks for pointing that out. Yes we should change it to be in the head. Added to my list.
“As high in the <head> as possible”
… 
I think they’re just being greedy there. A normal position in the head should be fine.
Depending on the kind of rating you’re after you can also use the ratings plugin, which automatically adds the average rating for a topic as structured data to the topic page html.
Example from a user here: Topic Ratings Plugin
@Gulshan_Kumar I updated our GTM support today so it adds the code to the head and the body according to the new instructions.
Currently I could not see star rating showing in SERP for above two post.
@Gulshan_Kumar - are you removing star rating in gtm or use any method?
The method which I suggested (using GTM+JSON-LD for star rating) works completely fine. You can continue to use it.
In my case, I intentionally removed from that post due to some personal reason. Nothing else.
