issue: [2717] coverage calculation improvements

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Description

As a user of coverage feature I faced need to see total and covered numbers of queries and mutations. So I decided to apply improvements.

Fixes # (2717)

Type of change

Please delete options that are not relevant.

[x] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[x] This change requires a documentation update

How Has This Been Tested?

I updated existing unit tests.

Test Environment:

OS:
@graphql-inspector/...:
NodeJS:

Checklist:

[ ] I have followed the CONTRIBUTING doc and the style guidelines of this project
[x] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[x] My changes generate no new warnings
[x] I have added tests that prove my fix is effective or that my feature works
[ ] New and existing unit tests pass locally with my changes
[ ] Any dependent changes have been merged and published in downstream modules

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

kamilkisiela / graphql-inspector