Add back alloc limits script with JSON mode (#2987)

Motivation:

We removed the alloc limits script, but it is still very useful. It just
couldn't produce JSON. So now it can!

Modifications:

Re-adds the alloc limits script.
Makes it produce JSON.

Result:

JSON can be produced from the CI output.

(cherry picked from commit 035d3a5)

Author: Lukasa

Diff for: dev/alloc-limits-from-test-output

@@ -0,0 +1,104 @@
+#!/bin/bash
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the SwiftNIO open source project
+##
+## Copyright (c) 2021 Apple Inc. and the SwiftNIO project authors
+## Licensed under Apache License v2.0
+##
+## See LICENSE.txt for license information
+## See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+##
+## SPDX-License-Identifier: Apache-2.0
+##
+##===----------------------------------------------------------------------===##
+
+# This script allows you to consume any Jenkins/alloc counter run output and
+# convert it into the right for for the docker-compose script.
+
+set -eu
+
+mode_flag=${1---docker-compose}
+
+function usage() {
+    echo >&1 "Usage: $0 [--docker-compose|--export|--json]"
+    echo >&1
+    echo >&1 "Example:"
+    echo >&1 "  # copy the output from the Jenkins CI into your clipboard, then"
+    echo >&1 "  pbpaste | $0 --docker-compose"
+}
+
+function die() {
+    echo >&2 "ERROR: $*"
+    exit 1
+}
+
+case "$mode_flag" in
+    --docker-compose)
+        mode=docker
+        ;;
+    --export)
+        mode="export"
+        ;;
+    --json)
+        mode=json
+        ;;
+    *)
+        usage
+        exit 1
+        ;;
+esac
+
+function allow_slack() {
+    raw="$1"
+    if [[ ! "$raw" =~ ^[0-9]+$ ]]; then
+        die "not a malloc count: '$raw'"
+    fi
+    if [[ "$raw" -lt 1000 ]]; then
+        echo "$raw"
+        return
+    fi
+
+    allocs=$raw
+    while true; do
+        allocs=$(( allocs + 1 ))
+        if [[ "$allocs" =~ [0-9]+00$ || "$allocs" =~ [0-9]+50$ ]]; then
+            echo "$allocs"
+            return
+        fi
+    done
+}
+
+json_blob="{"
+
+lines=$(grep -e "total number of mallocs" -e ".total_allocations" -e "export MAX_ALLOCS_ALLOWED_" | \
+    sed -e "s/: total number of mallocs: /=/g" \
+        -e "s/.total_allocations: /=/g" \
+        -e "s/info: /test_/g" \
+        -e "s/export MAX_ALLOCS_ALLOWED_/test_/g" | \
+    grep -Eo 'test_[a-zA-Z0-9_-]+=[0-9]+' | sort | uniq)
+
+while read -r info; do
+    test_name=$(echo "$info" | sed "s/test_//g" | cut -d= -f1 )
+    allocs=$(allow_slack "$(echo "$info" | cut -d= -f2 | sed "s/ //g")")
+    case "$mode" in
+        docker)
+            echo "      - MAX_ALLOCS_ALLOWED_$test_name=$allocs"
+            ;;
+        export)
+            echo "export MAX_ALLOCS_ALLOWED_$test_name=$allocs"
+            ;;
+        json)
+            json_blob="${json_blob}"$'\n'"    \"$test_name\": $allocs,"
+            ;;
+        *)
+            die "Unexpected mode: $mode"
+            ;;
+    esac
+done <<< "$lines"
+
+if [[ "$mode" == "json" ]]; then
+    json_blob=$(sed '$ s/,$//g' <<< "$json_blob")
+    json_blob+=$'\n'"}"
+    echo "$json_blob"
+fi