Truncate the message sent to sentry to max allowable chars

[max-b]

This attempts to address the issue described in https://github.com/getlantern/lantern-internal/issues/3620#issuecomment-616904350 where panics in the anacrolix/torrent library weren't getting reported to sentry.

From some random testing locally, it seems as though the stack trace we're attempting to sent to sentry is just too large and is causing the reporting to fail silently 😬 😬 (I think this is "documented" in getsentry/sentry-go#169)

I chose 8k as the max number of characters from basically looking at successful sentry reports and looking at the point at which they were truncated (I think that sentry does some kind of truncation before storing on their end).

I was able to get a panic inside a goroutine in the torrent library to be reported after making the change:
https://sentry.io/organizations/getlantern/issues/1633600374/?project=2222244

[max-b]

Oh I started investigating that issue on the sentry-go sdk repo and I think they may have actually just implemented this in v0.6.0.

Lemme double check that's the case and they we can just bump their version instead of this handrolled implementation...

[max-b]

Oh I started investigating that issue on the sentry-go sdk repo and I think they may have actually just implemented this in v0.6.0.

Lemme double check that's the case and they we can just bump their version instead of this handrolled implementation...

hmmm nope as far as I can tell it's still silently failing :/ I think if I'm reading the getsentry/sentry-go#168 correctly, it is just ensuring that the http integrations don't send too large of a body to the sentry client. Since we're manually sending the message I think we have to do that ourselves...

[myleshorton]

LGTM!

[max-b]

I'm gonna do a quick test of moving this into the BeforeSend function in the sentry options just to tidy it up a little and to make sure that we don't run into this again if we end up needing to use CaptureMessage somewhere else in the codebase..

[max-b]

Ok yeah I did a smol refactor and I like this way more. I tested that it still reports correctly: https://sentry.io/organizations/getlantern/issues/1633698328/?project=2222244&query=is%3Aunresolved

Thanks for being patient with me 😁 😁

[myleshorton]

Looks great @max-b -- gonna go ahead and merge!

[max-b]

Thanks!