refactor(balancer): get headers as hash identifier by ngx.var (#13063)

Before this PR we use `ngx.req.get_headers()` to get all possible header values as identifier,
but since nginx 1.23.0 or OpenResty  1.25.3.1 `ngx.var` can get a combined value for all header values 
with identical name (joined by comma), so I think that we could simplify these code. 

KAG-4572

kong/runloop/balancer/init.lua

@@ -131,10 +131,9 @@ local function get_value_to_hash(upstream, ctx)
       identifier = var.remote_addr
 
     elseif hash_on == "header" then
-      identifier = ngx.req.get_headers()[upstream[header_field_name]]
-      if type(identifier) == "table" then
-        identifier = table_concat(identifier)
-      end
+      -- since nginx 1.23.0/openresty 1.25.3.1
+      -- ngx.var will automatically combine all header values with identical name
+      identifier = var["http_" .. upstream[header_field_name]]
 
     elseif hash_on == "cookie" then
       identifier = var["cookie_" .. upstream.hash_on_cookie]

spec/02-integration/05-proxy/10-balancer/03-consistent-hashing_spec.lua

@@ -58,6 +58,45 @@ for _, strategy in helpers.each_strategy() do
             ["hashme"] = "just a value",
           })
           assert.are.equal(requests, oks)
+          assert.logfile().has.line("trying to get peer with value to hash: \\[just a value\\]")
+
+          -- collect server results; hitcount
+          -- one should get all the hits, the other 0
+          local count1 = server1:shutdown()
+          local count2 = server2:shutdown()
+
+          -- verify
+          assert(count1.total == 0 or count1.total == requests, "counts should either get 0 or ALL hits")
+          assert(count2.total == 0 or count2.total == requests, "counts should either get 0 or ALL hits")
+          assert(count1.total + count2.total == requests)
+        end)
+
+        it("hashing on multiple headers", function()
+          local requests = bu.SLOTS * 2 -- go round the balancer twice
+
+          bu.begin_testcase_setup(strategy, bp)
+          local upstream_name, upstream_id = bu.add_upstream(bp, {
+            hash_on = "header",
+            hash_on_header = "hashme",
+          })
+          local port1 = bu.add_target(bp, upstream_id, localhost)
+          local port2 = bu.add_target(bp, upstream_id, localhost)
+          local api_host = bu.add_api(bp, upstream_name)
+          bu.end_testcase_setup(strategy, bp)
+
+          -- setup target servers
+          local server1 = https_server.new(port1, localhost)
+          local server2 = https_server.new(port2, localhost)
+          server1:start()
+          server2:start()
+
+          -- Go hit them with our test requests
+          local oks = bu.client_requests(requests, {
+            ["Host"] = api_host,
+            ["hashme"] = { "1st value", "2nd value", },
+          })
+          assert.are.equal(requests, oks)
+          assert.logfile().has.line("trying to get peer with value to hash: \\[1st value, 2nd value\\]")
 
           -- collect server results; hitcount
           -- one should get all the hits, the other 0
@@ -95,6 +134,7 @@ for _, strategy in helpers.each_strategy() do
             ["nothashme"] = "just a value",
           })
           assert.are.equal(requests, oks)
+          assert.logfile().has.line("trying to get peer with value to hash: \\[\\]")
 
           -- collect server results; hitcount
           -- one should get all the hits, the other 0