Fix docs that use regexp anchors (#3504)

Remove/fix docs that use anchors in label regexp matches.

docs/querying/api.md

@@ -47,7 +47,7 @@ Names of query parameters that may be repeated end with `[]`.
 
 `<series_selector>` placeholders refer to Prometheus [time series
 selectors](basics.md#time-series-selectors) like `http_requests_total` or
-`http_requests_total{method=~"^GET|POST$"}` and need to be URL-encoded.
+`http_requests_total{method=~"(GET|POST)"}` and need to be URL-encoded.
 
 `<duration>` placeholders refer to Prometheus duration strings of the form
 `[0-9]+[smhdwy]`. For example, `5m` refers to a duration of 5 minutes.

docs/querying/basics.md

@@ -114,7 +114,7 @@ Label matchers can also be applied to metric names by matching against the inter
 `{__name__="http_requests_total"}`. Matchers other than `=` (`!=`, `=~`, `!~`) may also be used.
 The following expression selects all metrics that have a name starting with `job:`:
 
-    {__name__=~"^job:.*"}
+    {__name__=~"job:.*"}
 
 ### Range Vector Selectors
 

docs/querying/examples.md

@@ -29,11 +29,11 @@ Using regular expressions, you could select time series only for jobs whose
 name match a certain pattern, in this case, all jobs that end with `server`.
 Note that this does a substring match, not a full string match:
 
-    http_requests_total{job=~"server$"}
+    http_requests_total{job=~".*server"}
 
 To select all HTTP status codes except 4xx ones, you could run:
 
-    http_requests_total{status!~"^4..$"}
+    http_requests_total{status!~"4.."}
 
 ## Using functions, operators, etc.