[datastore] Rework metric producer upsertion (oxidecomputer#6759)

This is staged on top of oxidecomputer#6748 and is based on @bnaecker's [suggestion
on that
PR](oxidecomputer#6748 (comment)).

This changes `DataStore::producer_endpoint_create()` from "the caller
must specify the Oximeter ID they want to be assigned to the new
producer" (which required them to first list and choose an Oximeter) to
"the caller does _not_ provide an Oximeter ID, and instead we choose one
randomly in a CTE".

I don't think this _fixes_ oxidecomputer#323, per se, but maybe it reworks this area
enough that we can close it as no longer relevant?

nexus/db-model/src/oximeter_info.rs

@@ -9,7 +9,9 @@ use nexus_types::internal_api;
 use uuid::Uuid;
 
 /// A record representing a registered `oximeter` collector.
-#[derive(Queryable, Insertable, Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(
+    Queryable, Insertable, Selectable, Debug, Clone, Copy, PartialEq, Eq,
+)]
 #[diesel(table_name = oximeter)]
 pub struct OximeterInfo {
     /// The ID for this oximeter instance.