refactor metrics summary

[ChuckHend]

Metrics summary currently executes a query for every queue in pgmq.meta. Look into optimizing that - likely able to make 1-2 queries in total.

pgmq/src/metrics.rs

Lines 38 to 60 in 0fad4dc

	#[pg_extern(name = "metrics_all")]
	fn pgmq_metrics_all() -> Result<
	TableIterator<
	'static,
	(
	name!(queue_name, String),
	name!(queue_length, i64),
	name!(newest_msg_age_sec, Option<i32>),
	name!(oldest_msg_age_sec, Option<i32>),
	name!(total_messages, i64),
	name!(scrape_time, TimestampWithTimeZone),
	),
	>,
	crate::PgmqExtError,
	> {
	let all_queueus = listit()?;
	let mut results: MetricResult = Vec::new();
	for q in all_queueus {
	let q_results = query_summary(&q.0)?;
	results.extend(q_results);
	}
	Ok(TableIterator::new(results))
	}