feat: add additional handling of prometheus-cache in ngx_tpl.lua (#1212703)

[qihaiyan]

Description

{% if enabled_plugins["prometheus"] and not enabled_stream_plugins["prometheus"] then %}
    lua_shared_dict prometheus-metrics {* http.lua_shared_dict["prometheus-metrics"] *};
    -- add prometheus-cache here
    lua_shared_dict prometheus-cache {* http.lua_shared_dict["prometheus-cache"] *};
{% end  #%}

Maybe it's better to set prometheus-cache together with prometheus-metrics in ngx_tpl.lua

Only when use_apisix_base is set to true, will the prometheus-cache take effect.
But in order to integrate with QAT, i used a custom built openresty instead of apisix-runtime, so use_apisix_base is not true, and prometheus-cache is not setted in nginx.conf even if prometheus plugin is enabled.

Which issue(s) this PR fixes:

Fixes #12703

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Baoyuantop]

Additional testing is needed; the existing tests cannot verify whether prometheus-cache is correctly generated when use_apisix_base=false.

[qihaiyan]

I didn't find out how to set use_apisix_base=false in ut environment, it's setted in env.lua:

    local use_apisix_base = true
    if not or_info:find("apisix-nginx-module", 1, true) then
        use_apisix_base = false
    end

[Baoyuantop]

I checked the existing tests and didn't find anything related to use_apisix_base=false, so I think this is acceptable. Let's see what other maintainers think.