fix: limit the scope of the template average build time query to the last 100 (#19648)

This PR should resolve https://github.com/coder/internal/issues/719 by
limiting the `workspace_builds` rows selected by the query to the most
recent 100 builds of a template, as opposed to all builds in the last
30d. For our own internal templates with the most builds (1700-2000 in a
30d period) this should cut the query execution time by about 80%.

Unless we have some restriction on keeping the 30d period, contract
related or otherwise, this seems like a safe change to make. In addition
to the execution speed improvements it also means the memory for the
query is bounded as well.

If we want to keep a 30d time period for the avg build time value I
think it's worth exploring a purpose built solution such as histogram
structures where the build times could be bucketized by template ID as
they're observed.

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/metricscache/metricscache.go

@@ -101,16 +101,12 @@ func (c *Cache) refreshTemplateBuildTimes(ctx context.Context) error {
 	for _, template := range templates {
 		ids = append(ids, template.ID)
 
-		templateAvgBuildTime, err := c.database.GetTemplateAverageBuildTime(ctx, database.GetTemplateAverageBuildTimeParams{
-			TemplateID: uuid.NullUUID{
+		templateAvgBuildTime, err := c.database.GetTemplateAverageBuildTime(ctx,
+			uuid.NullUUID{
 				UUID:  template.ID,
 				Valid: true,
 			},
-			StartTime: sql.NullTime{
-				Time:  dbtime.Time(c.clock.Now().AddDate(0, 0, -30)),
-				Valid: true,
-			},
-		})
+		)
 		if err != nil {
 			return err
 		}