feat(observability-lib): add SortBy and SortDesc to LegendOptions (#1799)

* feat(observability-lib): add SortBy and SortDesc to LegendOptions

Expose the underlying Grafana SDK's VizLegendOptions sorting fields
to allow legend tables to be sorted by a specific calculation field
(e.g., "Last *", "Max") in ascending or descending order.

This enables dashboards to surface the most important series at the
top of the legend table, improving readability for error rate panels.

* refactor: simplify SortDesc handling per review feedback

Only set SortDesc when SortBy is specified, and pass the value directly
rather than hardcoding true.

* fix: address review comments

- Fix comment examples to match actual Grafana format ("Last *", "Max")
- Only set SortDesc when true to avoid unnecessary explicit false values

Author: leeyikjiun

observability-lib/grafana/panels.go

@@ -55,6 +55,8 @@ type LegendOptions struct {
 	Placement   common.LegendPlacement
 	DisplayMode common.LegendDisplayMode
 	Calcs       []string
+	SortBy      string // Sort legend by this field (e.g., "Last *", "Max", "Mean")
+	SortDesc    bool   // Sort in descending order
 }
 
 func newLegend(options *LegendOptions) *common.VizLegendOptionsBuilder {
@@ -77,6 +79,13 @@ func newLegend(options *LegendOptions) *common.VizLegendOptionsBuilder {
 
 	builder.DisplayMode(options.DisplayMode)
 
+	if options.SortBy != "" {
+		builder.SortBy(options.SortBy)
+		if options.SortDesc {
+			builder.SortDesc(true)
+		}
+	}
+
 	return builder
 }