Logs ingester and store queries boundaries. (#3849)

example:
```
 ts=2021-06-14T08:43:30.015895283Z caller=spanlogger.go:87 org_id=4336 traceID=0b6bb74a53efd739 level=debug msg="querying ingester" params="selector={stream=\"stdout\", pod=\"loki-canary-6  64d9d8d8f-99k22\"}, direction=BACKWARD, start=2021-06-14 08:17:03.037241855 +0000 UTC, end=2021-06-14 08:17:23.037241855 +0000 UTC, limit=1000, shards="
 ts=2021-06-14T08:43:30.016305592Z caller=spanlogger.go:87 org_id=4336 traceID=0b6bb74a53efd739 level=debug msg="querying store" params="selector={stream=\"stdout\", pod=\"loki-canary-664d  9d8d8f-99k22\"}, direction=BACKWARD, start=2021-06-14 08:17:03.037241855 +0000 UTC, end=2021-06-14 08:17:23.037241855 +0000 UTC, limit=1000, shards="
```

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

pkg/logql/ast.go

@@ -42,6 +42,14 @@ type SelectLogParams struct {
 	*logproto.QueryRequest
 }
 
+func (s SelectLogParams) String() string {
+	if s.QueryRequest != nil {
+		return fmt.Sprintf("selector=%s, direction=%s, start=%s, end=%s, limit=%d, shards=%s",
+			s.Selector, logproto.Direction_name[int32(s.Direction)], s.Start, s.End, s.Limit, strings.Join(s.Shards, ","))
+	}
+	return ""
+}
+
 // LogSelector returns the LogSelectorExpr from the SelectParams.
 // The `LogSelectorExpr` can then returns all matchers and filters to use for that request.
 func (s SelectLogParams) LogSelector() (LogSelectorExpr, error) {

pkg/querier/querier.go

@@ -107,7 +107,9 @@ func (q *Querier) SelectLogs(ctx context.Context, params logql.SelectLogParams)
 		}
 		newParams.Start = ingesterQueryInterval.start
 		newParams.End = ingesterQueryInterval.end
-
+		level.Debug(spanlogger.FromContext(ctx)).Log(
+			"msg", "querying ingester",
+			"params", newParams)
 		ingesterIters, err := q.ingesterQuerier.SelectLogs(ctx, newParams)
 		if err != nil {
 			return nil, err
@@ -119,7 +121,9 @@ func (q *Querier) SelectLogs(ctx context.Context, params logql.SelectLogParams)
 	if storeQueryInterval != nil {
 		params.Start = storeQueryInterval.start
 		params.End = storeQueryInterval.end
-
+		level.Debug(spanlogger.FromContext(ctx)).Log(
+			"msg", "querying store",
+			"params", params)
 		storeIter, err := q.store.SelectLogs(ctx, params)
 		if err != nil {
 			return nil, err