Similar to: #5979, removes the feature flag this was gated behind.

But with slightly different behaviour (in order of importane):

• Fixes a bug where the trace context of a transaction was modified, this can/should never happen. Transactions must always contain a valid trace context.
• No longer adds any metadata to the trace_id and span_id fields, when they are created. This was already inconsistent between no trace context and a partial trace context. If any metadata is desired, we should add it to the context instead (if it gets created). Metadata for missing attributes is preserved (although I don't think that's that important).
• Keeps the flag on the normalization config, to ease testing for the rest of Relay, not all normaliation tests need to be updated to assert a trace context now. This is also a good place to add some documentation.
• Fixes some tests that before asserted incorrect behaviour (e.g. contexts on transactions), adds more integration tests.