Add metrics to Horizon Lite

[Shaptic]

Epic: #4317

---

As part of Phase 3, we need metrics to effectively compare it to Sunset Horizon. This issue covers a few metrics:

• How long do indices take to build?
• How long do indices take to update in real time?
• How old is a request, and, how does it correlate with how long servicing it takes?
• How long different parts of the on-the-fly ingestion take:
  • index lookup,
  • ledger downloads,
  • ingestion itself, etc.

Some of these should be exposed as Prometheus metrics on the new /metrics endpoint while others are part of the logs.

[sreuland]

I'm working on these metrics initially:

http request horizon_http_requests_duration_seconds_sum and horizon_http_requests_duration_seconds_count
number of of ledgers traversed in http request horizon_http_requests_ledgers_fetched_sum and horizon_http_requests_ledgers_fetched_count

[edit - 07/22/22] - I'm tasked on few other things, I'm not active on this development, so if someone else wants to or already has taken these metric keys that I mentioned, all good, thanks!

[Shaptic]

I'm closing this in favor of leaving the one remaining issue open, #4528