fix(ruler): Initialize multi remote clients config (#7151)

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**What this PR does / why we need it**:
To be backward compatible with single client config.
```
ts=2022-09-14T08:24:04.549961234Z caller=memberlist_logger.go:74 level=debug msg="Initiating push/pull sync with:  10.132.18.167:7946"                                                                                                                     │
│ panic: assignment to entry in nil map                                                                                                                                                                                                                      │
│ goroutine 1 [running]:                                                                                                                                                                                                                                     │
│ github.com/grafana/loki/pkg/ruler.NewRuler({{{0xc00002a120}, {0x0, 0x0, 0x0}, {0x6400000, 0x6400000, {0x0, 0x0}, 0x0, 0x0, ...}, ...}, ...}, ...)                                                                                                          │
│     /src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ruler/ruler.go:21 +0x1b7   
```
**Which issue(s) this PR fixes**:
Fixes #<issue number>

**Special notes for your reviewer**:

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**Checklist**
- [ ] Documentation added
- [ ] Tests updated
- [ ] Is this an important fix or new feature? Add an entry in the
`CHANGELOG.md`.
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

pkg/ruler/config.go

@@ -106,4 +106,8 @@ func (c *RemoteWriteConfig) Clone() (*RemoteWriteConfig, error) {
 func (c *RemoteWriteConfig) RegisterFlags(f *flag.FlagSet) {
 	f.BoolVar(&c.Enabled, "ruler.remote-write.enabled", false, "Remote-write recording rule samples to Prometheus-compatible remote-write receiver.")
 	f.DurationVar(&c.ConfigRefreshPeriod, "ruler.remote-write.config-refresh-period", 10*time.Second, "Minimum period to wait between refreshing remote-write reconfigurations. This should be greater than or equivalent to -limits.per-user-override-period.")
+
+	if c.Clients == nil {
+		c.Clients = make(map[string]config.RemoteWriteConfig)
+	}
 }

pkg/ruler/config_test.go

@@ -0,0 +1,19 @@
+package ruler
+
+import (
+	"flag"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRemoteWriteConfig(t *testing.T) {
+	fs := flag.NewFlagSet("test", flag.ExitOnError)
+
+	r := RemoteWriteConfig{}
+	r.RegisterFlags(fs)
+
+	// assert new multi clients config is backward compatible if with single tenant.
+	// if not provided
+	assert.NotNil(t, r.Clients)
+}

pkg/ruler/ruler.go

@@ -4,6 +4,7 @@ import (
 	"github.com/go-kit/log"
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/prometheus/config"
 
 	"github.com/grafana/loki/pkg/logql"
 	ruler "github.com/grafana/loki/pkg/ruler/base"
@@ -18,6 +19,10 @@ func NewRuler(cfg Config, engine *logql.Engine, reg prometheus.Registerer, logge
 	}
 
 	if len(cfg.RemoteWrite.Clients) == 0 && cfg.RemoteWrite.Client != nil {
+		if cfg.RemoteWrite.Clients == nil {
+			cfg.RemoteWrite.Clients = make(map[string]config.RemoteWriteConfig)
+		}
+
 		cfg.RemoteWrite.Clients["default"] = *cfg.RemoteWrite.Client
 	}