Enable mTLS support for gNMI exporter.

[LarsxGitHub]

This CL extends the gNMI exporter with configuration support for mTLS.

MISC:

• added exemplary credentials to testdata. None of these credentials are used for anything besides testing.
• also fixes a previously broken test for gnmi/gnmi.go.

[robshakir]

@LarsxGitHub -- can you add the config to run the GitHub actions workflows here? It looks like go.yml is in the wrong place, so there are no checks running that validate the build.

[robshakir]

#17 will need to be submitted before this PR, since I made changes to the repo to require the CI checks pass before we merge.

[coveralls]

Pull Request Test Coverage Report for Build 11218708668

Details

• 70 of 93 (75.27%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.4%) to 77.474%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
gnmi/gnmi.go	3	4	75.0%
gnmi/security.go	67	77	87.01%
cmd/clio.go	0	12	0.0%

Files with Coverage Reduction	New Missed Lines	%
gnmi/gnmi.go	1	82.77%

Totals
Change from base Build 11217690156:	0.4%
Covered Lines:	368
Relevant Lines:	475

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💛 - Coveralls