app/vmselect: follow-up after 6a96fd8ed5

- Add `Active queries` chapter to VMUI docs
- Set `Content-Type: json` header inside promql.WriteActiveQueries() handler,
  in order to be consistent with other request handlers called at app/vmselect/main.go
- Pass the request to promql.WriteActiveQueries() handler, so it can change its output
  depending on the provided request params. This also improves consistency of
  promql.WriteActiveQueries() args with other request hanlers at app/vmselect/main.go

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/4653

docs/README.md

@@ -319,6 +319,7 @@ The UI allows exploring query results via graphs and tables. It also provides th
   - [Metrics explorer](#metrics-explorer) - automatically builds graphs for selected metrics; 
   - [Cardinality explorer](#cardinality-explorer) - stats about existing metrics in TSDB;
   - [Top queries](#top-queries) - shows most frequently executed queries;
+  - [Active queries](#active-queries) - shows currently executed queries;
 - Tools:
   - [Trace analyzer](#query-tracing) - playground for loading query traces in JSON format; 
   - [WITH expressions playground](https://play.victoriametrics.com/select/accounting/1/6a716b0f-38bc-4856-90ce-448fd713e3fe/prometheus/graph/#/expand-with-exprs) - test how WITH expressions work; 
@@ -363,6 +364,15 @@ See the [example VMUI at VictoriaMetrics playground](https://play.victoriametric
 * queries with the biggest average execution duration;
 * queries that took the most summary time for execution.
 
+## Active queries
+
+[VMUI](#vmui) provides `active queries` tab, which shows currently execute queries.
+It provides the following information per each query:
+
+- The query itself, together with the time range and step args passed to [/api/v1/query_range](https://docs.victoriametrics.com/keyConcepts.html#range-query).
+- The duration of the query execution.
+- The client address, who initiated the query execution.
+
 ## Metrics explorer
 
 [VMUI](#vmui) provides an ability to explore metrics exported by a particular `job` / `instance` in the following way:

docs/Single-server-VictoriaMetrics.md

@@ -327,6 +327,7 @@ The UI allows exploring query results via graphs and tables. It also provides th
   - [Metrics explorer](#metrics-explorer) - automatically builds graphs for selected metrics; 
   - [Cardinality explorer](#cardinality-explorer) - stats about existing metrics in TSDB;
   - [Top queries](#top-queries) - shows most frequently executed queries;
+  - [Active queries](#active-queries) - shows currently executed queries;
 - Tools:
   - [Trace analyzer](#query-tracing) - playground for loading query traces in JSON format; 
   - [WITH expressions playground](https://play.victoriametrics.com/select/accounting/1/6a716b0f-38bc-4856-90ce-448fd713e3fe/prometheus/graph/#/expand-with-exprs) - test how WITH expressions work; 
@@ -371,6 +372,15 @@ See the [example VMUI at VictoriaMetrics playground](https://play.victoriametric
 * queries with the biggest average execution duration;
 * queries that took the most summary time for execution.
 
+## Active queries
+
+[VMUI](#vmui) provides `active queries` tab, which shows currently execute queries.
+It provides the following information per each query:
+
+- The query itself, together with the time range and step args passed to [/api/v1/query_range](https://docs.victoriametrics.com/keyConcepts.html#range-query).
+- The duration of the query execution.
+- The client address, who initiated the query execution.
+
 ## Metrics explorer
 
 [VMUI](#vmui) provides an ability to explore metrics exported by a particular `job` / `instance` in the following way: