... meta description tag in google
To be clear, the description that appears in the SERPs in just that, a "description", it's not "the meta description tag". Google will show the contents of the meta description in the SERPs if it closely matches what the user is searching for, or there is no other text on the page. However, Google will often dynamically generate the description that appears in the SERPs from whatever the user is searching for and the textual content on the page.
In fact, pages will often omit the meta description entirely these days since Google does a good (if not better) job of dynamically generating a description. And sometimes it's not easy for the author of the page to create a suitable meta description anyway. (It also doesn't affect ranking, just takes up a few extra bytes and takes time to maintain.)
Is there anything that I can do to properly show the tag?
You may just be unlucky. Google will certainly show text like
<html-tag> in the SERPs. You just need to do a search for
iframe to see this. However, none of these pages appear to use the text
<iframe> in the meta description. In fact, some of these pages don't have a meta description tag (see above). Google is generating this description from the page text.
google also adds my domain name at the end of the title
It has often been suggested that this happens when your title tag is too short (eg. below the 55 char "optimum"), as closetnoc suggests in comments. This might well be part of it, however, this is certainly not the complete answer by any means, as there are plenty of short titles returned in the SERPs that don't have the domain appended to the end.
For example, currently, if I search for
hello on Google I get the following page titles returned:
Hello Magazine (14 chars)
Adele - Hello - YouTube (23 chars)
HELLO! | Facebook (17 chars)
hello network (13 chars)
Hello (5 chars)
About - Hello (13 chars)
From that example it's difficult to see how the length of the title has anything to do with it?