Use consistent keys for logs

Rule warnings are logged with numDropped=N while every other component uses num_dropped=N:

```
notifier/notifier.go:		level.Warn(n.logger).Log("msg", "Alert batch larger than queue capacity, dropping alerts", "num_dropped", d)
notifier/notifier.go:		level.Warn(n.logger).Log("msg", "Alert notification queue full, dropping alerts", "num_dropped", d)
storage/remote/write_handler.go:		_ = level.Warn(h.logger).Log("msg", "Error on ingesting out-of-order exemplars", "num_dropped", outOfOrderExemplarErrs)
rules/group.go:				level.Warn(logger).Log("msg", "Error on ingesting out-of-order result from rule evaluation", "num_dropped", numOutOfOrder)
rules/group.go:				level.Warn(logger).Log("msg", "Error on ingesting too old result from rule evaluation", "num_dropped", numTooOld)
rules/group.go:				level.Warn(logger).Log("msg", "Error on ingesting results from rule evaluation with different value but same timestamp", "num_dropped", numDuplicates)
scrape/scrape.go:		level.Warn(sl.l).Log("msg", "Error on ingesting out-of-order samples", "num_dropped", appErrs.numOutOfOrder)
scrape/scrape.go:		level.Warn(sl.l).Log("msg", "Error on ingesting samples with different value but same timestamp", "num_dropped", appErrs.numDuplicates)
scrape/scrape.go:		level.Warn(sl.l).Log("msg", "Error on ingesting samples that are too old or are too far into the future", "num_dropped", appErrs.numOutOfBounds)
scrape/scrape.go:		level.Warn(sl.l).Log("msg", "Error on ingesting out-of-order exemplars", "num_dropped", appErrs.numExemplarOutOfOrder)
```

Rename numDropped to num_dropped for consistency.

Signed-off-by: Łukasz Mierzwa <l.mierzwa@gmail.com>

rules/group.go

@@ -546,13 +546,13 @@ func (g *Group) Eval(ctx context.Context, ts time.Time) {
 				}
 			}
 			if numOutOfOrder > 0 {
-				level.Warn(logger).Log("msg", "Error on ingesting out-of-order result from rule evaluation", "numDropped", numOutOfOrder)
+				level.Warn(logger).Log("msg", "Error on ingesting out-of-order result from rule evaluation", "num_dropped", numOutOfOrder)
 			}
 			if numTooOld > 0 {
-				level.Warn(logger).Log("msg", "Error on ingesting too old result from rule evaluation", "numDropped", numTooOld)
+				level.Warn(logger).Log("msg", "Error on ingesting too old result from rule evaluation", "num_dropped", numTooOld)
 			}
 			if numDuplicates > 0 {
-				level.Warn(logger).Log("msg", "Error on ingesting results from rule evaluation with different value but same timestamp", "numDropped", numDuplicates)
+				level.Warn(logger).Log("msg", "Error on ingesting results from rule evaluation with different value but same timestamp", "num_dropped", numDuplicates)
 			}
 
 			for metric, lset := range g.seriesInPreviousEval[i] {