chore(2step): copy over scaffolding from fault-detector

packages/two-step-monitor/.depcheckrc

+ignores: [
+  "@babel/eslint-parser",
+  "@typescript-eslint/parser",
+  "eslint-plugin-import",
+  "eslint-plugin-unicorn",
+  "eslint-plugin-jsdoc",
+  "eslint-plugin-prefer-arrow",
+  "eslint-plugin-react",
+  "@typescript-eslint/eslint-plugin",
+  "eslint-config-prettier",
+  "eslint-plugin-prettier",
+  "chai"
+]

packages/two-step-monitor/.env.example

+# URL for an L1 RPC provider, used to query L2 output proposals
+TWO_STEP_MONITOR__L1_RPC_PROVIDER=
+
+# URL for an L2 RPC provider, used to query canonical L2 state
+TWO_STEP_MONITOR__L2_RPC_PROVIDER=
+
+TWO_STEP_MONITOR__HOSTNAME=
+TWO_STEP_MONITOR__PORT=
+
+TWO_STEP_MONITOR__START_BATCH_INDEX=
+TWO_STEP_MONITOR__LOOP_INTERVAL_MS=

packages/two-step-monitor/.eslintrc.js

+module.exports = {
+  extends: '../../.eslintrc.js',
+}

packages/two-step-monitor/.gitignore

+node_modules/
+build/

packages/two-step-monitor/.lintstagedrc.yml

+"*.{ts,js}":
+  - eslint

packages/two-step-monitor/.prettierrc.js

+module.exports = {
+  ...require('../../.prettierrc.js'),
+};

packages/two-step-monitor/LICENSE

+(The MIT License)
+
+Copyright 2020-2021 Optimism
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

packages/two-step-monitor/README.md

+# @eth-optimism/two-step-monitor
+
+[![codecov](https://codecov.io/gh/ethereum-optimism/optimism/branch/develop/graph/badge.svg?token=0VTG7PG7YR&flag=two-step-monitor-tests)](https://codecov.io/gh/ethereum-optimism/optimism)
+
+The `two-step-monitor` is a simple service for detecting discrepancies between withdrawals created on L2, and
+withdrawals proven on L1.
+
+## Installation
+
+Clone, install, and build the Optimism monorepo:
+
+```
+git clone https://github.com/ethereum-optimism/optimism.git
+yarn install
+yarn build
+```
+
+## Running the service
+
+Copy `.env.example` into a new file named `.env`, then set the environment variables listed there.
+Once your environment variables have been set, run the service via:
+
+```
+yarn start
+```
+
+## Ports
+
+- API is exposed at `$TWO_STEP_MONITOR__HOSTNAME:$TWO_STEP_MONITOR__PORT/api`
+- Metrics are exposed at `$TWO_STEP_MONITOR__HOSTNAME:$TWO_STEP_MONITOR__PORT/metrics`
+- `$TWO_STEP_MONITOR__HOSTNAME` defaults to `0.0.0.0`
+- `$TWO_STEP_MONITOR__PORT` defaults to `7300`
+
+## What this service does
+
+The `two-step-monitor` detects when a withdrawal is proven on L1, and verifies that a corresponding withdrawal
+has been created on L2.
+
+We export a series of Prometheus metrics that you can use to trigger alerting when issues are detected.
+Check the list of available metrics via `yarn start --help`:
+
+```sh
+> yarn start --help
+yarn run v1.22.19
+$ ts-node ./src/service.ts --help
+Usage: service [options]
+
+Options:
+  --l1rpcprovider    Provider for interacting with L1 (env: TWO_STEP_MONITOR__L1_RPC_PROVIDER)
+  --l2rpcprovider    Provider for interacting with L2 (env: TWO_STEP_MONITOR__L2_RPC_PROVIDER)
+  --port             Port for the app server (env: TWO_STEP_MONITOR__PORT)
+  --hostname         Hostname for the app server (env: TWO_STEP_MONITOR__HOSTNAME)
+  -h, --help         display help for command
+
+Metrics:
+  l1_node_connection_failures   Number of times L1 node connection has failed (type: Gauge)
+  l2_node_connection_failures   Number of times L2 node connection has failed (type: Gauge)
+  metadata                      Service metadata (type: Gauge)
+  unhandled_errors              Unhandled errors (type: Counter)
+
+Done in 2.19s.
+```

packages/two-step-monitor/hardhat.config.ts

+import { HardhatUserConfig } from 'hardhat/types'
+
+// Hardhat plugins
+import '@nomiclabs/hardhat-ethers'
+import '@nomiclabs/hardhat-waffle'
+
+const config: HardhatUserConfig = {
+  mocha: {
+    timeout: 50000,
+  },
+}
+
+export default config

packages/two-step-monitor/package.json

+{
+  "private": true,
+  "name": "@eth-optimism/two-step-monitor",
+  "version": "0.5.0",
+  "description": "[Optimism] Service for detecting faulty L2 output proposals",
+  "main": "dist/index",
+  "types": "dist/index",
+  "files": [
+    "dist/*"
+  ],
+  "scripts": {
+    "start": "ts-node ./src/service.ts",
+    "test": "hardhat test",
+    "test:coverage": "nyc hardhat test && nyc merge .nyc_output coverage.json",
+    "build": "tsc -p tsconfig.json",
+    "clean": "rimraf dist/ ./tsconfig.tsbuildinfo",
+    "lint": "yarn lint:fix && yarn lint:check",
+    "pre-commit": "lint-staged",
+    "lint:fix": "yarn lint:check --fix",
+    "lint:check": "eslint . --max-warnings=0"
+  },
+  "keywords": [
+    "optimism",
+    "ethereum",
+    "fault",
+    "detector"
+  ],
+  "homepage": "https://github.com/ethereum-optimism/optimism/tree/develop/packages/two-step-monitor#readme",
+  "license": "MIT",
+  "author": "Optimism PBC",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/ethereum-optimism/optimism.git"
+  },
+  "devDependencies": {
+    "@nomiclabs/hardhat-ethers": "^2.0.6",
+    "@nomiclabs/hardhat-waffle": "^2.0.3",
+    "@types/chai": "^4.3.1",
+    "chai-as-promised": "^7.1.1",
+    "ethers": "^5.7.0",
+    "hardhat": "^2.9.6",
+    "ts-node": "^10.9.1"
+  }
+}

packages/two-step-monitor/src/index.ts

+export * from './service'

packages/two-step-monitor/src/service.ts

+export const todo = 'implement me'

packages/two-step-monitor/test/setup.ts

+import chai = require('chai')
+import chaiAsPromised from 'chai-as-promised'
+
+// Chai plugins go here.
+chai.use(chaiAsPromised)
+
+const should = chai.should()
+const expect = chai.expect
+
+export { should, expect }

packages/two-step-monitor/tsconfig.json

+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "outDir": "./dist"
+  },
+  "include": [
+    "package.json",
+    "src/**/*"
+  ]
+}