fix(dbrpv2): reflect match count correctly

[gavincabbage]

This PR fixes a bug in the V2 DBRP Mapping Service where the number of matching mappings was returned incorrectly because the length of the match list was calculated before it was populated.

An illustration of the bug on the Go playground:

func main() {

	n, a := func() (int, []string) {
		var b []string
		return len(b), func() []string {
			for i := 0; i < 3; i++ {
				b = append(b, fmt.Sprintf("%d", i))
			}
			return b
		}()
	}()

	fmt.Printf("n: %d, a: %v\n", n, a)
}

n: 0, a: [0 1 2]

Tests are also updated and have been confirmed to fail without the fix in place.

• Well-formatted commit messages
• Rebased/mergeable
• Tests pass

[stuartcarnie]

Awesome – great find and thanks for fixing!