insights: support querying just the "last" aggregated data point (pie charts, etc.)

Created by: slimsag

Many insights will only need to render the "last" data point. Instead of the frontend querying the most recent time range (which would work) we can allow the frontend to pass a Limit option. The DB store already supports it, it's just not exposed over GraphQL: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@61e8253febdee226c940a81c467affb0d4d0b809/-/blob/enterprise/internal/insights/resolvers/insight_series_resolver.go#L42:5