promtail: register reload metrics after running options (#7561)

**What this PR does / why we need it**:
This allows the reload metrics to be sent to a custom registerer, which
is required for Grafana Agent to be able to use the Promtail code.

**Which issue(s) this PR fixes**:
N/A 

**Special notes for your reviewer**:
Is it possible for us to cherry-pick this into k121? That's the version
we're trying to update Grafana Agent to.

**Checklist**
- [x] Reviewed the `CONTRIBUTING.md` guide
- [x] Documentation added
- [x] Tests updated
- [x] `CHANGELOG.md` updated
- [x] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

clients/pkg/promtail/promtail.go

@@ -82,6 +82,13 @@ func New(cfg config.Config, newConfig func() (*config.Config, error), metrics *c
 		metrics: metrics,
 		dryRun:  dryRun,
 	}
+	for _, o := range opts {
+		// todo (callum) I don't understand why I needed to add this check
+		if o == nil {
+			continue
+		}
+		o(promtail)
+	}
 	err := promtail.reg.Register(reloadSuccessTotal)
 	if err != nil {
 		return nil, fmt.Errorf("error register prometheus collector reloadSuccessTotal :%w", err)
@@ -90,13 +97,6 @@ func New(cfg config.Config, newConfig func() (*config.Config, error), metrics *c
 	if err != nil {
 		return nil, fmt.Errorf("error register prometheus collector reloadFailTotal :%w", err)
 	}
-	for _, o := range opts {
-		// todo (callum) I don't understand why I needed to add this check
-		if o == nil {
-			continue
-		}
-		o(promtail)
-	}
 	err = promtail.reloadConfig(&cfg)
 	if err != nil {
 		return nil, err