ref(grpc): migrate server to context scope API

[giortzisg]

Description

This migrates the grpc integration to use the new scopes API.

Issues

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[sentry]

Bug: When a parent span exists, a new transaction is created but not added to the context or finished, leading to lost child span data.
_{Severity: MEDIUM}

Suggested Fix

When owned == false, avoid creating a new, orphaned transaction. The logic should either retrieve and use the existing transaction from the parent context or, if a new transaction is strictly necessary, ensure it is properly finished. The current implementation, which creates and then discards a transaction without finishing it, should be corrected to prevent data loss.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: grpc/server.go#L88-L91

Potential issue: In the `startServerTransaction` function, when a parent span already
exists (the `owned == false` case), a new transaction is created but is never finished,
nor is it added to the context returned to the handler. This creates an inconsistent
state. If the handler or subsequent logic creates child spans using this new, unfinished
`transaction` object, those child spans are recorded but will never be sent to Sentry
because their parent transaction is never completed. This results in silently lost
tracing data for nested operations.