[receivers/hostmetrics] add conntrack metrics

[codeboten]

For linux only, adding metrics about connection tracking table.

[github-actions]

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

[mwear]

LGTM

[dmitryax]

Why do we want these metrics to be emitted by default? they seems like a good candidate for optional metrics

[codeboten]

Why do we want these metrics to be emitted by default? they seems like a good candidate for optional metrics

What's the best way to determine when a metric should be on by default vs not?

[dmitryax]

We don't have any strict guidelines to determine that, probably we should have it. Usually it's just up to code owner.

I usually follow the following reasoning:

• The default set of metrics is enough to monitor a system at whole to determine its healthiness.
• Optional metrics are additional nice-to-have metrics that are not essential to determine system healthiness.

We also should take into consideration how stable a receiver is. If it has been used in production for some time, adding default metrics can affect downsteam backends that will unexpectedly receive additional load caused by added metrics with another collector upgrade.

[codeboten]

Why do we want these metrics to be emitted by default? they seems like a good candidate for optional metrics

I agree, have disabled them by default. PTAL

[TylerHelmuth]

We don't have any strict guidelines to determine that, probably we should have it. Usually it's just up to code owner.

I usually follow the following reasoning:

• The default set of metrics is enough to monitor a system at whole to determine its healthiness.
• Optional metrics are additional nice-to-have metrics that are not essential to determine system healthiness.

We also should take into consideration how stable a receiver is. If it has been used in production for some time, adding default metrics can affect downsteam backends that will unexpectedly receive additional load caused by added metrics with another collector upgrade.

These thoughts would be awesome in the "Adding metrics to existing receivers" section of the contributing guide 👀