Some of the Prometheus metrics seem to be unused

Hi there, me again :).

I decided to set up Prometheus monitoring because I was hoping to get alerted when/if I stop receiving e-mails frequently (as that has usually shown to be a problem in the past).

I was quite confused by two points:

* the `maddy_*` prometheus metrics don't seem to appear in the `/metrics` output until they actually get incremented (maybe this is intentional? But as a Prometheus noob I would have liked to know on the documentation page before wondering if I missed something)
* some of the metrics seem to be unused — for example `completedSMTPTransactions` which doesn't seem to have a corresponding increment line

```
❯ rg completedSMTPTransactions
internal/endpoint/smtp/metrics.go
33: completedSMTPTransactions = prometheus.NewCounterVec(
83: prometheus.MustRegister(completedSMTPTransactions)
```

I can have a crack at these problems if you don't think I'm wrong.

I would also like to add some more metrics — for example on the number of e-mails that get successfully sent over LMTP (perhaps excluding ones received from local users ... hmm), so that I can have a very clear number of 'received e-mails' that actually make it to Dovecot rather than the queues getting deadlocked (which seems to have bitten me every few months on older versions).
Does that sound reasonable?

從 internal/endpoint/smtp/metrics.go 開始，接著在儲存庫中搜尋 completedSMTPTransactions 和其他 maddy_* 指標的使用情況。重現 issue 中所描述的 /metrics 行為，並判斷哪些指標實際上會被更新；當未使用指標的相關發現已獲得解決，且預期的指標可見性已記錄或驗證後，即表示完成。

由索引模型根據 Issue 內容生成。