Create SQL functions to send a metrics from postgres to open telemetry collector

[teoincontatto]

Create an SQL function to send a metrics from postgres to open telemetry collector for each type of metric supported by OTEL protocol:

• CUMULATIVE
• GAUGE
• MEASURE

The functions should also accept a set of labels. See https://github.com/open-telemetry/oteps/blob/main/text/metrics/0049-metric-label-set.md

[asotolongo]

To send the metrics to opentelemetry is possible using the function SELECT pgotel_counter('URL_OPENTELEMETRY', 'counter_name', VALUE);, so far the process will be necessary be complete in less 2 seconds, due to this configuration, but can be changed.

a pg_cron entry can generate this sending process

[fabriziomello]

@asotolongo the problem is that the current architecture of Metrics in OTEL SDK it will continue sending the metric continuously after each 2 seconds.

[asotolongo]

@fabriziomello but if I remember well, once the connection closes, the extension don't send the metrics, from my POV pg_cron connect does the task and disconnects (maybe 2 seconds is too short to send all metrics in one connection, it can be increased)

[teoincontatto]

Seems you can force a flush of a metric using Meterprovider::Forceflush() (see open-telemetry/opentelemetry-cpp#2516).

Maybe if you set a very high export_interval_millis and force a flush in the C function it will send the metric instantly.