Update code_coverage.md to include example for calculating coverage of entire Go project

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What does this MR do?

The current published version of this doc provides an example on how to integrate Go's test coverage tool. Due to the Gitlab CI/CD only capturing the last matching line for a given coverage regular expression in the job definition and the example command for Go resulting in the following output

ok      gitlab.com/example/foo 0.008s  coverage: 100.0% of statements
ok      gitlab.com/example/bar 0.008s  coverage: 100.0% of statements
ok      gitlab.com/example/fizz 0.008s  coverage: 95.0% of statements
ok      gitlab.com/example/buzz 0.008s  coverage: 80.0% of statements

Will result in the captured code coverage being 80.0% by Gitlab because the last line of the output reports the coverage for one of the packages in the project.

To have Gitlab capture the coverage for the entire project then the following two commands need to be executed:

• go test -coverprofile=cover.profile -cover ./...
• go tool cover -func cover.profile

The output of the go tool command will look like

gitlab.com/example/foo/foo.go:22:     Foo          100.0%
gitlab.com/example/bar/bar.go:11:     Bar          100.0%
gitlab.com/example/fizz/fizz.go:7:    Fizz         95.0%
gitlab.com/example/buzz/buzz.go:13:   Buzz         80.0%
total:                                (statements) 70.0%

So the following regular expression can be used on this new output to capture the project's total coverage /total:\s+(statements)\s+\d+.\d+%/ .

The changes in this MR would like to update the docs to provide the correct example for capturing the coverage of the entire project when using Go as the language for the project. For Gitlab let me know if you would like to special case the examples for Go as I have with the current revision between single package and whole project or would you just like to have one of the examples used instead for Go.

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