Alerting: Switch to snappy-compressed-protobuf for outgoing push requests to Loki (#65077)

* Encode with snappy, always

* JSON encoder type

* Headers

* Copy labels formatter from promtail

* Implement snappy-proto encoding

* Create encoder interface, test both encoders, choose snappy-proto by default

* Make encoder configurable at the LokiCfg level

* Export both encoders

* Touch up comment and tests

* Drop unnecessary conversions after move to plain strings to appease linter

pkg/services/ngalert/state/historian/loki_http_test.go

@@ -117,6 +117,7 @@ func TestLokiHTTPClient_Manual(t *testing.T) {
 		client := newLokiClient(LokiConfig{
 			ReadPathURL:  url,
 			WritePathURL: url,
+			Encoder:      JsonEncoder{},
 		}, NewRequester(), metrics.NewHistorianMetrics(prometheus.NewRegistry()), log.NewNopLogger())
 
 		// Unauthorized request should fail against Grafana Cloud.
@@ -144,6 +145,7 @@ func TestLokiHTTPClient_Manual(t *testing.T) {
 			WritePathURL:      url,
 			BasicAuthUser:     "<your_username>",
 			BasicAuthPassword: "<your_password>",
+			Encoder:           JsonEncoder{},
 		}, NewRequester(), metrics.NewHistorianMetrics(prometheus.NewRegistry()), log.NewNopLogger())
 
 		// When running on prem, you might need to set the tenant id,
@@ -259,6 +261,7 @@ func createTestLokiClient(req client.Requester) *httpLokiClient {
 	cfg := LokiConfig{
 		WritePathURL: url,
 		ReadPathURL:  url,
+		Encoder:      JsonEncoder{},
 	}
 	met := metrics.NewHistorianMetrics(prometheus.NewRegistry())
 	return newLokiClient(cfg, req, met, log.NewNopLogger())