feat(stats): sparklines for connections and transactions or queries per second (#1165)

[ZhuchkaTriplesix]

Refs #1165

What was wrong

The Overview polled the server every few seconds but showed only the current numbers, so a spike between two looks was invisible.

Changes

• MetricHistory (core/stats/metric_history.dart): the last 120 samples per metric (ten minutes at the 5 s poll), kept in memory while the Overview is open. A counter's change between two samples becomes a rate per second; a counter that goes down (a restart) gives none.
• QueryaSparkline (shared/widgets/querya_sparkline.dart): a line of the samples; the tooltip names the latest value and its time; draws nothing until there are two samples.
• PostgreSQL: active connections and transactions per second (from xact_commit across databases), in the Connections card.
• MySQL: connected threads and queries per second (from Questions), in the Connections card.
• The history is fed from the numbers each poll already fetched, so no query is added. The view rebuilds on every poll sample, so the lines move.
• Tests: the capacity, the rate from a counter, the restart case, read-only series.

Not in this PR

• The acceptance test "no extra queries per poll with a counting fake service" is not written; the counting is by construction (the history reads the fetched stats only).
• The sparkline widget itself is not tested.
• Cache hit ratio, locks and waiting sessions are not added, and MongoDB is not changed. Those need the driver queries the issue names.
• The history is kept while the view is mounted; it is not kept across closing the Overview.

Not verified locally

Tests were not run locally, per the project rule.