fix error assertions in the tracer

[marten-seemann]

Fixes #233.

errors.Is and errors.As still confuses me. What was happening here was that the case for the version negotiation errors didn't recognize a quic.VersionNegotiationError because... well, I don't really know.
I assume it's because a *quic.VersionNegotiationError has (exported) fields, and errors.Is was looking for an exact match, not just a type match. This is what errors.As apparently does.
The same applies to a bunch of other error types as well.

With this PR, we should be able to properly increment the counter for version negotiation errors (and the other error types), instead of creating a new Prometheus counter for every error.

[mvdan]

Indeed, Is is roughly equivalent to err == ErrSentinel, and As is roughly equivalent to err.(ErrType), but both supporting error wrapping.

FYI, if you don't actually need to use the variables like applicationErr, you could keep the code as it was before, like:

case errors.As(e, &quic.ApplicationError{}):

Though most times one uses As, it's also to inspect the details of the typed error. Up to you.

[marten-seemann]

FYI, if you don't actually need to use the variables like applicationErr, you could keep the code as it was before, like:

case errors.As(e, &quic.ApplicationError{}):

Though most times one uses As, it's also to inspect the details of the typed error. Up to you.

Unfortunately, that doesn't work (and is one of the reasons I find working with errors.As so annoying). You need a pointer to a pointer:

# github.com/libp2p/go-libp2p-quic-transport
./tracer_metrics.go:243:7: second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type

Or is there something I'm missing?