Prometheus metrics with axum middleware in payjoin-service

[spacebear21]

Some proposed metrics (from https://github.com/orgs/payjoin/discussions/775)

• request counters
  • segregated by OHTTP gateway, OHTTP target resource and backwards compatibility endpoints
  • by response type: 2xx, 4xx, 5xx
    • GET: 200 OK vs 202 ACCEPTED
  • unique shortid counters (using hyperloglog?)
    • weekly, daily & hourly HLL entries? custom ultraloglog or hyperloglog based metric, analogous to histogram in prometheus parlance, unique shortids in a time period
  • number of in flight (DB blocked long polling) requests
• number of entries in DB (v1 vs v2)
• total payload length for v1 requests
• performance counters?