[receiver/tlscheck] Add the attribute `tlscheck.x509.san` to metrics

[atoulme]

Component(s)

receiver/tlscheck

Is your feature request related to a problem? Please describe.

This would be an optional additional attribute on top of what cn offers.

Describe the solution you'd like

This is particularly useful if cn is not present.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• receiver/tlscheck: @atoulme @michael-burt

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