Add /api/v1/format_query API endpoint for formatting queries (#11036)

* Add /api/v1/format_query API endpoint for formatting queries

This uses the formatting functionality introduced in
https://github.com/prometheus/prometheus/pull/10544.

I've chosen "query" instead of "expr" in both the endpoint and parameter
names to stay consistent with the existing API endpoints. Otherwise, I
would have preferred to use the term "expr".

Signed-off-by: Julius Volz <julius.volz@gmail.com>

* Add docs for /api/v1/format_query endpoint

Signed-off-by: Julius Volz <julius.volz@gmail.com>

* Add note that formatting expressions removes comments

Signed-off-by: Julius Volz <julius.volz@gmail.com>

docs/querying/api.md

@@ -206,6 +206,35 @@ $ curl 'http://localhost:9090/api/v1/query_range?query=up&start=2015-07-01T20:10
 }
 ```
 
+## Formatting query expressions
+
+The following endpoint formats a PromQL expression in a prettified way:
+
+```
+GET /api/v1/format_query
+POST /api/v1/format_query
+```
+
+URL query parameters:
+
+- `query=<string>`: Prometheus expression query string.
+
+You can URL-encode these parameters directly in the request body by using the `POST` method and
+`Content-Type: application/x-www-form-urlencoded` header. This is useful when specifying a large
+query that may breach server-side URL character limits.
+
+The `data` section of the query result is a string containing the formatted query expression. Note that any comments are removed in the formatted string.
+
+The following example formats the expression `foo/bar`:
+
+```json
+$ curl 'http://localhost:9090/api/v1/format_query?query=foo/bar'
+{
+   "status" : "success",
+   "data" : "foo / bar"
+}
+```
+
 ## Querying metadata
 
 Prometheus offers a set of API endpoints to query metadata about series and their labels.

web/api/v1/api.go

@@ -323,6 +323,9 @@ func (api *API) Register(r *route.Router) {
 	r.Get("/query_exemplars", wrapAgent(api.queryExemplars))
 	r.Post("/query_exemplars", wrapAgent(api.queryExemplars))
 
+	r.Get("/format_query", wrapAgent(api.formatQuery))
+	r.Post("/format_query", wrapAgent(api.formatQuery))
+
 	r.Get("/labels", wrapAgent(api.labelNames))
 	r.Post("/labels", wrapAgent(api.labelNames))
 	r.Get("/label/:name/values", wrapAgent(api.labelValues))
@@ -428,6 +431,15 @@ func (api *API) query(r *http.Request) (result apiFuncResult) {
 	}, nil, res.Warnings, qry.Close}
 }
 
+func (api *API) formatQuery(r *http.Request) (result apiFuncResult) {
+	expr, err := parser.ParseExpr(r.FormValue("query"))
+	if err != nil {
+		return invalidParamError(err, "query")
+	}
+
+	return apiFuncResult{expr.Pretty(0), nil, nil, nil}
+}
+
 func extractQueryOpts(r *http.Request) *promql.QueryOpts {
 	return &promql.QueryOpts{
 		EnablePerStepStats: r.FormValue("stats") == "all",

web/api/v1/api_test.go

@@ -930,6 +930,20 @@ func testEndpoints(t *testing.T, api *API, tr *testTargetRetriever, es storage.E
 			},
 			errType: errorBadData,
 		},
+		{
+			endpoint: api.formatQuery,
+			query: url.Values{
+				"query": []string{"foo+bar"},
+			},
+			response: "foo + bar",
+		},
+		{
+			endpoint: api.formatQuery,
+			query: url.Values{
+				"query": []string{"invalid_expression/"},
+			},
+			errType: errorBadData,
+		},
 		{
 			endpoint: api.series,
 			query: url.Values{