make evictions_low_residence_duration_metric_threshold per-tenant (#3949)

Before this patch, if a tenant would override its eviction_policy
setting to use a lower LayerAccessThreshold::threshold than the
`evictions_low_residence_duration_metric_threshold`, the evictions done
for that tenant would count towards the
`evictions_with_low_residence_duration` metric.

That metric is used to identify pre-mature evictions, commonly triggered
by disk-usage-based eviction under disk pressure.

We don't want that to happen for the legitimate evictions of the tenant
that overrides its eviction_policy.

So, this patch
- moves the setting into TenantConf
- adds test coverage
- updates the staging & prod yamls

Forward Compatibility:
Software before this patch will ignore the new tenant conf field and use
the global one instead.
So we can roll back safely.

Backward Compatibility:
Parsing old configs with software as of this patch will fail in
`PageServerConf::parse_and_validate` with error 
`unrecognized pageserver option 'evictions_low_residence_duration_metric_threshold'`
if the option is still present in the global section.
We deal with this by updating the configs in Ansible.

fixes https://github.com/neondatabase/neon/issues/3940

libs/pageserver_api/src/models.rs

@@ -135,6 +135,7 @@ pub struct TenantCreateRequest {
     // For now, this field is not even documented in the openapi_spec.yml.
     pub eviction_policy: Option<serde_json::Value>,
     pub min_resident_size_override: Option<u64>,
+    pub evictions_low_residence_duration_metric_threshold: Option<String>,
 }
 
 #[serde_as]
@@ -181,6 +182,7 @@ pub struct TenantConfigRequest {
     // For now, this field is not even documented in the openapi_spec.yml.
     pub eviction_policy: Option<serde_json::Value>,
     pub min_resident_size_override: Option<u64>,
+    pub evictions_low_residence_duration_metric_threshold: Option<String>,
 }
 
 impl TenantConfigRequest {
@@ -202,6 +204,7 @@ impl TenantConfigRequest {
             trace_read_requests: None,
             eviction_policy: None,
             min_resident_size_override: None,
+            evictions_low_residence_duration_metric_threshold: None,
         }
     }
 }