Nexus's `OpContext` could store the request ID when it's available

[bnaecker]

This came up in conversations on #8566. Nexus uses the OpContext to keep a bunch of relevant information about what it's doing. One major reason for this is that we can trace the operations Nexus takes in response to things like an external API request. But we don't currently store the request ID we get as part of the Dropshot request anywhere! We should.

Note that we don't always have a request ID, such as if we've created a context object for background tasks or saga execution. But for internal / external API requests, we do, and we could cache that in the OpKind inside the context object.

[bnaecker]

This is already done, actually. op_context_for_external_api calls OpContext::load_request_metadata, which inserts the request ID into the map of key-value pairs. We can use that in the DTrace probes that prompted this.