docs: Add note about counter_server_query_cache_hit metric (#4946)

Fixes: #4389

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>

docs/content/rest-api.md

@@ -2044,6 +2044,12 @@ OPA currently supports the following query performance metrics:
 - **timer_rego_module_parse_ns**: time taken (in nanoseconds) to parse the input policy module.
 - **timer_rego_module_compile_ns**: time taken (in nanoseconds) to compile the loaded policy modules.
 - **timer_server_handler_ns**: time take (in nanoseconds) to handle the API request.
+- **counter_server_query_cache_hit**: number of cache hits for the query.
+
+The `counter_server_query_cache_hit` counter gives an indication about whether OPA creates a new Rego query
+or it uses a pre-processed query which holds some prepared state to serve the API request. A pre-processed query will be
+faster to evaluate since OPA will not have to re-parse or compile it. Hence, when the query is served from the cache
+`timer_rego_query_parse_ns` and `timer_rego_query_compile_ns` timers will be omitted from the reported performance metrics.
 
 OPA also supports query instrumentation. To enable query instrumentation,
 specify the `instrument=true` query parameter when executing the API call.