fix(otlp): Send span name as an attribute

[gggritso]

This is functionally a very similar PR to https://github.com/getsentry/relay/pull/4540/ but for the string properly span.name. In the linked PR, Matt opted to add kind as a top-level field in SpanV1. This causes the span buffer to map that property to sentry.kind. In this PR I'm taking an easier approach, and writing the sentry.name property straight to the tags.

I figured this is simpler but the same result, but let me know if I should introduce name to the schema instead!