[Lambda][CloudWatch] Memory Usage as a CloudWatch Metric

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**Tell us about your request**
Adding Lambda memory usage as a CloudWatch metric would enable users to create alarms on memory usage, allowing proactive adjustments to memory allocation before encountering errors. It would also simplify monitoring this critical parameter by making memory usage directly visible in the Monitoring section of the Lambda console.

This enhancement would improve visualization of memory usage and facilitate better handling of memory-related issues (e.g., insufficient memory) before they result in errors.

**Which service(s) is this request for?**
AWS Lambda

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently, there are no direct metrics for memory usage in CloudWatch. Users must rely on CloudWatch Logs Insights queries to extract memory usage data, which adds complexity and is less user-friendly.

**Are you currently working around this issue?**
To monitor memory usage, we need to navigate to Monitoring, then CloudWatch Logs, and execute a Logs Insights query such as:
```
filter @type = "REPORT"
| stats max(@memorySize / 1000 / 1000) as provisionedMemoryMB,
min(@maxMemoryUsed / 1000 / 1000) as smallestMemoryRequestMB,
avg(@maxMemoryUsed / 1000 / 1000) as avgMemoryUsedMB,
max(@maxMemoryUsed / 1000 / 1000) as maxMemoryUsedMB,
provisionedMemoryMB - maxMemoryUsedMB as overProvisionedMB
```
Alternatively, memory usage information can be found in "Recent invocations" and "Most expensive invocations in GB-seconds (memory assigned * billed duration)." However, these methods are not the most user-friendly or efficient for monitoring memory usage.

**Additional context**
N/A

**Attachments**
N/A

issue では、リポジトリのファイル、テスト、コードのエントリポイントについて言及されていません。まず、ここで説明されている AWS Lambda と CloudWatch のメトリクス機能を確認し、必要なメトリクス定義と統合経路を特定してください。Lambda のメモリ使用量を CloudWatch メトリクスとして利用でき、監視とアラームに使用できれば完了です。

索引モデルが issue の本文から書いたものです。