healthchecks: Handle no such host from Network Check and show MetaApiConnErr as WARNING.

[franciscovalentecastro]

When verifying connection to endpoints during Network Check execution (see network_check.go), if the endpoint doesn't exist it shows the following error (see #1630) :

[Network Check] Result: ERROR, Detail: Get "http://metadata.google.internal/": dial tcp *:

This PR does the following changes :

• Handle no such host from Network Check and present FAILURE (log and monitoring api endpoints) or WARNING (not required endpoints for Ops Agent execution) when appropiate.
• Show MetaApiConnErr as WARNING (not fatal error).

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[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.