config: add --disablegraph

[joostjager]

lndmon is known (reported by multiple users) to cause high cpu load problems for lnd. This is cause by the DescribeGraph call which is really too heavy for frequent scraping.

This PR adds a command line flag to disable the graph metrics.

[joostjager]

@carlaKC how about this one? I believe almost every user of lndmon has run into this

[carlaKC]

Out of curiosity, what hardware are you running on where you get issues with graph metrics?

I think this is worth adding, because we're likely not going to get around to streamlining the graph metrics with high prio.

[joostjager]

It is running on a dual-core VM. But I don't think that a full graph fetch at a typical scrape interval is ever going to work.

[carlaKC]

But I don't think that a full graph fetch at a typical scrape interval is ever going to work.

Yeah, definitely not ideal to poll the whole graph. Would be great to move a few of the metrics over to subscriptions, but it's difficult to find the time when it's mostly working.