feat: support get last peer connection cached

[tzssangglass]

Signed-off-by: tzssangglass [email protected]

Fix: https://konghq.atlassian.net/browse/FTI-5616

[tzssangglass]

apply this feat in Kong maybe like:

diff --git a/kong/init.lua b/kong/init.lua
index d37a08325..667b99e30 100644
--- a/kong/init.lua
+++ b/kong/init.lua
@@ -95,6 +95,7 @@ local wasm = require "kong.runloop.wasm"
 local reports = require "kong.reports"
 local pl_file = require "pl.file"
 local req_dyn_hook = require "kong.dynamic_hook"
+local peer_conn = require("resty.kong.peer_conn")
 
 
 local kong             = kong
@@ -1278,6 +1279,7 @@ function Kong.balancer()
     -- record failure data
     local previous_try = tries[try_count - 1]
     previous_try.state, previous_try.code = get_last_failure()
+    previous_try.cached = peer_conn.get_last_peer_connection_cached()
 
     -- Report HTTP status for health checks
     local balancer_instance = balancer_data.balancer
@@ -1330,6 +1332,8 @@ function Kong.balancer()
     if retries > 0 then
       set_more_tries(retries)
     end
+
+    tries[try_count].cached = 0
   end

adding a "cached" field in the tries structure of the log to indicate that the connection used for this retry was obtained from the connection pool.

looks like

"tries": [
        {
            "ip": "10.42.8.161",
            "state": "failed",
            "port": 443,
            "code": 502,
            "balancer_start": 1701368939054,
			"cached": 1
            "balancer_latency": 0
        }

[chronolaw]

�Does It only work in http subsystem? Should we support it in stream subsystem?

Another style suggestion: we should always have two blank lines to separate function and other code block.

[tzssangglass]

Should we support it in stream subsystem?

I think we don't need to support in stream subsystem, because it is a patch of ngx_http_lua_balancer, not ngx_stream_lua_balancer.

[tzssangglass]

ping @dndx , PTAL

[dndx]

Could you write a README doc for this new feature? It is hard to understand what it does without one.

[tzssangglass]

Could you write a README doc for this new feature? It is hard to understand what it does without one.

add

[dndx]

@tzssangglass very nice documentation, I merged your PR