Pass the correct context to staleness Appender (#10588)

OTel Collector prints the following error when a target disappears:

```
2022-04-13T14:20:24.932-0400	warn	scrape/scrape.go:1408	Stale append failed	{"kind": "receiver", "name": "prometheus", "scrape_pool": "beep-boop", "target": "http://localhost:9090/metrics", "error": "transaction aborted"}
```

This `transaction aborted` error is returned by the custom appender that is
used by the collector when the context of the appender is cancelled:
b7bf11174e/receiver/prometheusreceiver/internal/otlp_transaction.go (L81-L82)

We call `endOfRunStaleness` after `sl.stop()` which cancels `sl.ctx`.
The other `.Appender()` calls use `parentCtx` for the same reason.

This hasn't come up so far because Prometheus' Appender implementation just
ignores the context passed.

Signed-off-by: Goutham Veeramachaneni <gouthamve@gmail.com>

scrape/scrape.go

@@ -1379,7 +1379,8 @@ func (sl *scrapeLoop) endOfRunStaleness(last time.Time, ticker *time.Ticker, int
 	// Call sl.append again with an empty scrape to trigger stale markers.
 	// If the target has since been recreated and scraped, the
 	// stale markers will be out of order and ignored.
-	app := sl.appender(sl.ctx)
+	// sl.context would have been cancelled, hence using sl.parentCtx.
+	app := sl.appender(sl.parentCtx)
 	var err error
 	defer func() {
 		if err != nil {
@@ -1393,7 +1394,7 @@ func (sl *scrapeLoop) endOfRunStaleness(last time.Time, ticker *time.Ticker, int
 	}()
 	if _, _, _, err = sl.append(app, []byte{}, "", staleTime); err != nil {
 		app.Rollback()
-		app = sl.appender(sl.ctx)
+		app = sl.appender(sl.parentCtx)
 		level.Warn(sl.l).Log("msg", "Stale append failed", "err", err)
 	}
 	if err = sl.reportStale(app, staleTime); err != nil {