chore: add variables benchmark (#304)

<!-- The PR description should answer 2 (maybe 3) important questions:
-->

### What

Noticed we're not reusing connections in variable queries. Making a
benchmark for it before making the fix.

### How

Add a benchmark that does a query with 5 variables, meaning 5 queries.

benchmarks/component/benchmarks/select-variables.js

@@ -0,0 +1,89 @@
+import { check } from "k6";
+import http from "k6/http";
+import { newSummaryHandler } from "../common.js";
+
+const testid = "select_variables";
+const agentSocket = __ENV.AGENT_SOCKET || "localhost:8100";
+const url = `http://${agentSocket}/query`;
+const data = {
+  collection: "Album",
+  query: {
+    fields: {
+      Title: {
+        type: "column",
+        column: "Title",
+        arguments: {},
+      },
+    },
+    where: {
+      type: "binary_comparison_operator",
+      column: {
+        type: "column",
+        name: "Title",
+        path: [],
+      },
+      operator: {
+        type: "other",
+        name: "_like",
+      },
+      value: {
+        type: "variable",
+        name: "search",
+      },
+    },
+  },
+  arguments: {},
+  collection_relationships: {},
+  variables: [
+    {
+      search: "%Garage%",
+    },
+    {
+      search: "%Good%",
+    },
+    {
+      search: "%Rock%",
+    },
+    {
+      search: "%Dog%",
+    },
+    {
+      search: "%Log%",
+    },
+  ],
+};
+
+export default function () {
+  const response = http.post(url, JSON.stringify(data), {
+    headers: {
+      "Content-Type": "application/json",
+    },
+  });
+
+  check(response, {
+    "status is 200": (r) => r.status == 200,
+  });
+}
+
+export const handleSummary = newSummaryHandler(testid);
+
+export const options = {
+  tags: {
+    testid,
+  },
+  scenarios: {
+    short_sustained: {
+      executor: "constant-vus",
+      vus: 100,
+      duration: "10s",
+    },
+  },
+  thresholds: {
+    checks: [
+      {
+        threshold: "rate == 1",
+        abortOnFail: true,
+      },
+    ],
+  },
+};