## Summary
Enables metrics for the `pgxpool` that is used by the PostgreSQL
databroker backend.
Metrics are updated at 1s interval.
Will add the following metric output in the regular Prometheus
`/metrics` endpoint:
```
# HELP pomerium_pgxpool_acquire_duration_nanoseconds_total Total duration of all successful acquires from the pool in nanoseconds.
# TYPE pomerium_pgxpool_acquire_duration_nanoseconds_total counter
pomerium_pgxpool_acquire_duration_nanoseconds_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 5.1058702e+07
# HELP pomerium_pgxpool_acquired_connections Number of currently acquired connections in the pool.
# TYPE pomerium_pgxpool_acquired_connections gauge
pomerium_pgxpool_acquired_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_acquires_total Cumulative count of successful acquires from the pool.
# TYPE pomerium_pgxpool_acquires_total counter
pomerium_pgxpool_acquires_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 91
# HELP pomerium_pgxpool_canceled_acquires_total Cumulative count of acquires from the pool that were canceled by a context.
# TYPE pomerium_pgxpool_canceled_acquires_total counter
pomerium_pgxpool_canceled_acquires_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_constructing_connections_milliseconds Number of connections with construction in progress in the pool.
# TYPE pomerium_pgxpool_constructing_connections_milliseconds gauge
pomerium_pgxpool_constructing_connections_milliseconds{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_empty_acquire_total Cumulative count of successful acquires from the pool that waited for a resource to be released or constructed because the pool was empty.
# TYPE pomerium_pgxpool_empty_acquire_total counter
pomerium_pgxpool_empty_acquire_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 6
# HELP pomerium_pgxpool_idle_connections Number of currently idle connections in the pool.
# TYPE pomerium_pgxpool_idle_connections gauge
pomerium_pgxpool_idle_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 5
# HELP pomerium_pgxpool_max_connections Maximum size of the pool.
# TYPE pomerium_pgxpool_max_connections gauge
pomerium_pgxpool_max_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 10
# HELP pomerium_pgxpool_max_idle_destroys_total Cumulative count of connections destroyed because they exceeded MaxConnectionsIdleTime.
# TYPE pomerium_pgxpool_max_idle_destroys_total counter
pomerium_pgxpool_max_idle_destroys_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_max_lifetime_destroys_total Cumulative count of connections destroyed because they exceeded MaxConnectionsLifetime.
# TYPE pomerium_pgxpool_max_lifetime_destroys_total counter
pomerium_pgxpool_max_lifetime_destroys_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_new_connections_total Cumulative count of new connections opened.
# TYPE pomerium_pgxpool_new_connections_total counter
pomerium_pgxpool_new_connections_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 6
# HELP pomerium_pgxpool_total_connections Total number of resources currently in the pool. The value is the sum of ConstructingConnections, AcquiredConnections, and IdleConnections.
# TYPE pomerium_pgxpool_total_connections gauge
pomerium_pgxpool_total_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 5
```
## Related issues
<!-- For example...
- #159
-->
## User Explanation
<!-- How would you explain this change to the user? If this
change doesn't create any user-facing changes, you can leave
this blank. If filled out, add the `docs` label -->
## Checklist
- [x] reference any related issues
- [x] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review
This also replaces instances where we manually write "return ctx.Err()"
with "return context.Cause(ctx)" which is functionally identical, but
will also correctly propagate cause errors if present.
* wip
* storage: add filtering to SyncLatest
* don't increment the record version, so intermediate changes are requested
* databroker: add support for query filtering
* fill server and record version
* postgres: databroker storage backend
* wip
* serialize puts
* add test
* skip tests for macos
* add test
* return error from protojson
* set data
* exclude postgres from cover tests
