make metric_mtx type and device correct

[quancs]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

fix #534

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #533 (31198e8) into master (d6112a2) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #533   +/-   ##
=====================================
  Coverage      95%    95%           
=====================================
  Files         132    132           
  Lines        4676   4680    +4     
=====================================
+ Hits         4457   4461    +4     
  Misses        219    219           

[SkafteNicki]

LGTM