[compute_tools] Add /insights endpoint to compute_ctl (#3704)

This commit adds a basic HTTP API endpoint that allows scraping the
`pg_stat_statements` data and getting a list of slow queries. New
insights like cache hit rate and so on could be added later.

Extension `pg_stat_statements` is checked / created only if compute
tries to load the corresponding shared library. The latter is configured
by control-plane and currently covered with feature flag.

Co-authored by Eduard Dyckman (bird.duskpoet@gmail.com)

compute_tools/src/http/api.rs

@@ -33,6 +33,13 @@ async fn routes(req: Request<Body>, compute: &Arc<ComputeNode>) -> Response<Body
             Response::new(Body::from(serde_json::to_string(&compute.metrics).unwrap()))
         }
 
+        // Collect Postgres current usage insights
+        (&Method::GET, "/insights") => {
+            info!("serving /insights GET request");
+            let insights = compute.collect_insights().await;
+            Response::new(Body::from(insights))
+        }
+
         (&Method::POST, "/check_writability") => {
             info!("serving /check_writability POST request");
             let res = crate::checker::check_writability(compute).await;

compute_tools/src/http/openapi_spec.yaml

@@ -10,12 +10,12 @@ paths:
   /status:
     get:
       tags:
-      - "info"
+      - Info
       summary: Get compute node internal status
       description: ""
       operationId: getComputeStatus
       responses:
-        "200":
+        200:
           description: ComputeState
           content:
             application/json:
@@ -25,27 +25,43 @@ paths:
   /metrics.json:
     get:
       tags:
-      - "info"
+      - Info
       summary: Get compute node startup metrics in JSON format
       description: ""
       operationId: getComputeMetricsJSON
       responses:
-        "200":
+        200:
           description: ComputeMetrics
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ComputeMetrics"
 
+  /insights:
+    get:
+      tags:
+      - Info
+      summary: Get current compute insights in JSON format
+      description: |
+        Note, that this doesn't include any historical data
+      operationId: getComputeInsights
+      responses:
+        200:
+          description: Compute insights
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ComputeInsights"
+
   /check_writability:
     post:
       tags:
-      - "check"
+      - Check
       summary: Check that we can write new data on this compute
       description: ""
       operationId: checkComputeWritability
       responses:
-        "200":
+        200:
           description: Check result
           content:
             text/plain:
@@ -96,6 +112,15 @@ components:
           type: string
           description: Text of the error during compute startup, if any
 
+    ComputeInsights:
+      type: object
+      properties:
+        pg_stat_statements:
+          description: Contains raw output from pg_stat_statements in JSON format
+          type: array
+          items:
+            type: object
+
     ComputeStatus:
       type: string
       enum: