feat(tracing): Allow storing span metadata

[lobsterkatie]

This adds span metadata to the TransactionMetadata type and the default value given to the metadata property in the Transaction constructor. I chose to add a property to TransactionMetadata rather than create a new SpanMetadata type/property which would live on the Span class because by doing the former, we don't have to worry about looping over the spans and deleting the metadata before the transaction is sent - everything in transaction.metadata (which becomes event.sdkProcessingMetadata) is already cleared when the envelope is created.

In order to not have to assert on its existence every time it's used, I made spanMetadata a required property of TransactionMetadata. In order to satisfy that required-ness, a few other things had to happen:

• It had to be initialized in the Transaction constructor. (Note that there should never be incoming span metadata because the only span at that point is the transaction itself, and its metadata can and does already live in the main metadata object. This means there's no need to do span: metadata.span || {} when initializing it. )
• The metadata passed to the constructor in the transaction context needed to become a partial (which it effectively already was, since until now all properties have been optional).
• The metadata passed to setMetadata similarly had to become a partial.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.36 KB (0%)
@sentry/browser - ES5 CDN Bundle (minified)	59.92 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.93 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	52.79 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.69 KB (0%)
@sentry/browser - Webpack (minified)	64.13 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.71 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.02 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.81 KB (+0.04% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.06 KB (+0.03% 🔺)