Add postgres connection pool instrumentation

[evankanderson]

I was looking at https://opentelemetry.io/ecosystem/registry/?s=sql&component=&language=go and saw that splunk had some pq bindings.

Per https://pkg.go.dev/github.com/signalfx/splunk-otel-go/instrumentation/database/sql/splunksql#section-readme, this only includes the following metrics, no distributions / error rates on SQL queries:

db.client.connections.usage ({connection}) - The number of connections that are currently in state described by the state attribute
db.client.connections.max ({connection}) - The maximum number of open connections allowed
db.client.connections.wait_time (ms) - The time it took to obtain an open connection from the pool

[evankanderson]

Sample metrics from Docker Compose:

# HELP mediator_db_client_connections_max The maximum number of open connections allowed
# TYPE mediator_db_client_connections_max gauge
mediator_db_client_connections_max{otel_scope_name="github.com/signalfx/splunk-otel-go/instrumentation/database/sql/splunksql",otel_scope_version="1.9.0",pool_name="dbname=mediator host=postgres port=5432 sslmode=disable user=postgres"} 0
# HELP mediator_db_client_connections_usage The number of connections that are currently in state described by the state attribute
# TYPE mediator_db_client_connections_usage gauge
mediator_db_client_connections_usage{otel_scope_name="github.com/signalfx/splunk-otel-go/instrumentation/database/sql/splunksql",otel_scope_version="1.9.0",pool_name="dbname=mediator host=postgres port=5432 sslmode=disable user=postgres",state="idle"} 1
mediator_db_client_connections_usage{otel_scope_name="github.com/signalfx/splunk-otel-go/instrumentation/database/sql/splunksql",otel_scope_version="1.9.0",pool_name="dbname=mediator host=postgres port=5432 sslmode=disable user=postgres",state="used"} 0
# HELP mediator_db_client_connections_wait_time_milliseconds The time it took to obtain an open connection from the pool
# TYPE mediator_db_client_connections_wait_time_milliseconds gauge
mediator_db_client_connections_wait_time_milliseconds{otel_scope_name="github.com/signalfx/splunk-otel-go/instrumentation/database/sql/splunksql",otel_scope_version="1.9.0",pool_name="dbname=mediator host=postgres port=5432 sslmode=disable user=postgres"} 0