Audit log: more ergonomic audit log call

[david-crespo]

Followup to #7339. Related to #8811.

Right now the audit log calls are a bit verbose and error-prone because they require two manual calls, and they coexist in a rather noisy way with the latency-tracking function call. This is the cleanest-looking one:

omicron/nexus/src/external_api/http_entrypoints.rs

Lines 919 to 947 in 6ab7e96

	async fn project_create(
	rqctx: RequestContext<ApiContext>,
	new_project: TypedBody<params::ProjectCreate>,
	) -> Result<HttpResponseCreated<Project>, HttpError> {
	let apictx = rqctx.context();
	let handler = async {
	let opctx =
	crate::context::op_context_for_external_api(&rqctx).await?;
	let nexus = &apictx.context.nexus;
	let audit = nexus.audit_log_entry_init(&opctx, &rqctx).await?;
	let result = async {
	let project = nexus
	.project_create(&opctx, &new_project.into_inner())
	.await?;
	Ok(HttpResponseCreated(project.into()))
	}
	.await;
	let _ =
	nexus.audit_log_entry_complete(&opctx, &audit, &result).await;
	result
	};
	apictx
	.context
	.external_latencies
	.instrument_dropshot_handler(&rqctx, handler)
	.await
	}

Dropshot deliberately does not support middleware, which would let us do this kind of thing automatically outside of the handlers. Finding a more ergonomic and less noisy way of doing the audit logging and latency logging might require a declarative macro. Or maybe not.

Ideally we would be able to do both the audit log call and the latency tracking call in a single function that takes our inner handler function, and it uses trait constraints on the result type to extract an ID from it if it is present.