lib/flagutil/dict: properly update default value in case there is no key value set (#7211)

### Describe Your Changes

If a dict flag has only one value without a prefix it is supposed to
replace default value.

Previously, when flag was set to `-flag=2` and the default value in
`NewDictInt` was set to 1 the resulting value for any `flag.Get()` call
would be 1 which is not expected.

This commit updates default value for the flag in case there is only one
entry for flag and the entry is a number without a key.

This affects cluster version and specifically `replicationFactor` flag
usage with vmstorage [node
groups](https://docs.victoriametrics.com/cluster-victoriametrics/#vmstorage-groups-at-vmselect).
Previously, the following configuration would effectively be ignored:
```
/path/to/vmselect \
 -replicationFactor=2 \
 -storageNode=g1/host1,g1/host2,g1/host3 \
 -storageNode=g2/host4,g2/host5,g2/host6 \
 -storageNode=g3/host7,g3/host8,g3/host9
```

Changes from this PR will force default value for `replicationFactor`
flag to be set to `2` which is expected as the result of this
configuration.


---------

Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>

docs/changelog/CHANGELOG.md

@@ -32,6 +32,8 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * BUGFIX: [vmalert-tool](https://docs.victoriametrics.com/vmalert-tool/): reduce the initial health check interval for datasource. This reduces the time spent on evaluating rules by vmalert-tool. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6970).
 * BUGFIX: [vmalert-tool](https://docs.victoriametrics.com/vmalert-tool/): allow specifying empty labels list `labels: '{}'` in the same way as promtool does. This improves compatibility between these tools.
 
+* BUGFIX: [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): properly apply replication factor when storage node groups are used and replication factor is configured via global value such as `-replicationFactor=2`. Previously, global replication factor was ignored for storage node groups. See [these docs](https://docs.victoriametrics.com/cluster-victoriametrics/#vmstorage-groups-at-vmselect) for more information about storage groups configuration.
+
 ## [v1.104.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.104.0)
 
 Released at 2024-10-02

lib/flagutil/dict.go

@@ -56,6 +56,7 @@ func (di *DictInt) Set(value string) error {
 		di.kvs = append(di.kvs, kIntValue{
 			v: v,
 		})
+		di.defaultValue = v
 		return nil
 	}
 	for _, x := range values {

lib/flagutil/dict_test.go

@@ -109,5 +109,5 @@ func TestDictIntGet(t *testing.T) {
 	f("foo:42", "", 123, 123)
 	f("foo:42", "foo", 123, 42)
 	f("532", "", 123, 532)
-	f("532", "foo", 123, 123)
+	f("532", "foo", 123, 532)
 }