feat: bare-bones /v1/utilization (#6831)

PR adds a simple at most 1Hz refreshed informational API for querying
pageserver utilization. In this first phase, no actual background
calculation is performed. Instead, the worst possible score is always
returned. The returned bytes information is however correct.

Cc: #6835
Cc: #5331

pageserver/src/http/openapi_spec.yml

@@ -1379,6 +1379,25 @@ paths:
               schema:
                 $ref: "#/components/schemas/ServiceUnavailableError"
 
+  /v1/utilization:
+    get:
+      description: |
+        Returns the pageservers current utilization and fitness score for new tenants.
+
+      responses:
+        "200":
+            description: Pageserver utilization and fitness score
+            content:
+              application/json:
+                schema:
+                  $ref: "#/components/schemas/PageserverUtilization"
+        "500":
+          description: Generic operation error
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Error"
+
 components:
   securitySchemes:
     JWT:
@@ -1691,6 +1710,33 @@ components:
           type: string
           enum: [past, present, future, nodata]
 
+    PageserverUtilization:
+      type: object
+      required:
+        - disk_usage_bytes
+        - free_space_bytes
+        - utilization_score
+      properties:
+        disk_usage_bytes:
+          type: integer
+          format: int64
+          minimum: 0
+          description: The amount of disk space currently utilized by layer files.
+        free_space_bytes:
+          type: integer
+          format: int64
+          minimum: 0
+          description: The amount of usable disk space left.
+        utilization_score:
+          type: integer
+          format: int64
+          minimum: 0
+          maximum: 9223372036854775807
+          default: 9223372036854775807
+          description: |
+            Lower is better score for how good this pageserver would be for the next tenant.
+            The default or maximum value can be returned in situations when a proper score cannot (yet) be calculated.
+
     Error:
       type: object
       required: