feat(tracing): Add transaction name to tracestate value

[lobsterkatie]

This adds transaction name to the tracetate value.

Doing this has two known limitations, which, though we've discussed them, I'm adding here for posterity:

1. Adding this data puts us over the character limit for tracestate values listed in the W3C spec (256 characters). To be fair, depending on the data, we might already be over this limit with the previous addition of user . For reference:

   Given the tracestate data:

   {
     trace_id: "12312012123120121231201212312012",
     environment: dogpark,
     release: off.leash.park,
     public_key: dogsarebadatkeepingsecrets,
     user: {id: 12312013, segment: "bigs"},
     transaction: /interactions/other-dogs/new-dog/
   }
   

   tracestate with without either: 196 characters
   tracestate with user data: 256 characters
   tracestate with transaction: 264 characters
   tracestate with both: 324 characters

2. This data may change and/or get added to the scope after the tracestate value has been calculated, making it impossible to sample based on those attributes. This is especially a problem for transaction name, which in some frameworks isn't set to its final value until the transaction ends. This poses the added problem that the transaction name in its raw, un-finalized form may contain PII, because it is often the raw URL as opposed to the parameterized one (so, /users/maisey/tricks/ rather than /users/:username/tricks/). More work needs to be done to investigate whether the final transaction name can be set earlier in any/all of the frameworks where this poses a problem. (For instance, it is a known problem in our Express integration, but not yet clear if it is a problem in any Python frameworks.)

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.9 KB (+0.01% 🔺)
@sentry/browser - Webpack	22.86 KB (0%)
@sentry/react - Webpack	22.89 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	30.07 KB (+0.05% 🔺)