Add db entry metrics

[zealsham]

this pr addresses the db entries part of #941 , it adds db entries metrics seperated by version (v1/v2). For now it's just simple incremental counter seperated by v1/v2.

Pull Request Checklist

Please confirm the following before requesting review:

• I have disclosed my use of
  AI
  in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Pull Request Test Coverage Report for Build 23269894458

Details

• 90 of 96 (93.75%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 84.215%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-mailroom/src/lib.rs	9	15	60.0%

Totals
Change from base Build 23193941891:	0.09%
Covered Lines:	10729
Relevant Lines:	12740

---

💛 - Coveralls

[spacebear21]

cACK, but I have a suggestion for a different approach.

Now that the directory lives in payjoin-mailroom, we should be able to wrap a Metrics wrapper around the DB following a middleware pattern. Something like this:

// db/mod.rs

  /// Db decorator that records write metrics.
  #[derive(Clone)]
  pub struct MetricsDb<D: Db> {
      inner: D,
      metrics: MetricsService,
  }

  impl<D: Db> Db for MetricsDb<D> {
      type OperationalError = D::OperationalError;

      async fn post_v2_payload(
          &self,
          mailbox_id: &ShortId,
          data: Vec<u8>,
      ) -> Result<Option<()>, Error<Self::OperationalError>> {
          let result = self.inner.post_v2_payload(mailbox_id, data).await?;
          self.metrics.record_db_entry("2");
          Ok(result)
      }
// ...
}

// lib.rs
struct Services {
      directory: directory::Service<MetricsDb<DbServiceAdapter>>,
      relay: ohttp_relay::Service,
      metrics: MetricsService,
      // ...
  }

async fn init_directory(
      config: &Config,
      sentinel_tag: SentinelTag,
      metrics: MetricsService,
  ) -> anyhow::Result<directory::Service<MetricsDb<DbServiceAdapter>>> {
      let files_db = FilesDb::init(config.timeout, config.storage_dir.clone()).await?;
      files_db.spawn_background_prune().await;
      let db = MetricsDb::new(DbServiceAdapter::new(files_db), metrics);
      // ...
  }

This keeps metrics out of the directory and follows a more modular approach in line with tower patterns.

[spacebear21]

This clones the metrics service, that would result in two separate instances of MetricsService tracking different things independently right? We should probably pass some type of reference to ensure there is only one source of truth.