fix(tracing): allow passing nil to span:set_attribute

* passing a nil value to `span:set_attribute` is a NOOP if the attribute
  does not already exists, else it means unsetting that attribute
* considered a fix because previously this was causing a stack trace
  when the DNS spans were created without a port

kong/pdk/tracing.lua

@@ -296,11 +296,12 @@ end
 --
 -- @function span:set_attribute
 -- @tparam string key
--- @tparam string|number|boolean value
+-- @tparam string|number|boolean|nil value
 -- @usage
 -- span:set_attribute("net.transport", "ip_tcp")
 -- span:set_attribute("net.peer.port", 443)
 -- span:set_attribute("exception.escaped", true)
+-- span:set_attribute("unset.this", nil)
 function span_mt:set_attribute(key, value)
   -- key is decided by the programmer, so if it is not a string, we should
   -- error out.
@@ -309,7 +310,7 @@ function span_mt:set_attribute(key, value)
   end
 
   local vtyp = type(value)
-  if vtyp ~= "string" and vtyp ~= "number" and vtyp ~= "boolean" then
+  if vtyp ~= "string" and vtyp ~= "number" and vtyp ~= "boolean" and vtyp ~= "nil" then
     -- we should not error out here, as most of the caller does not catch
     -- errors, and they are hooking to core facilities, which may cause
     -- unexpected behavior.

spec/01-unit/26-tracing/01-tracer_pdk_spec.lua

@@ -189,11 +189,20 @@ describe("Tracer PDK", function()
       assert.has_no.error(function () span:finish() end)
     end)
 
-    it("fails set_attribute", function ()
+    it("set_attribute validation", function ()
       local span = c_tracer.start_span("meow")
 
+      -- nil value is allowed as a noop
       span:set_attribute("key1")
-      assert.spy(log_spy).was_called_with(ngx.ERR, match.is_string())
+      assert.spy(log_spy).was_not_called_with(ngx.ERR, match.is_string())
+      assert.is_nil(span.attributes["key1"])
+
+      span:set_attribute("key1", "value1")
+      assert.equal("value1", span.attributes["key1"])
+
+      -- nil value unsets the attribute
+      span:set_attribute("key1")
+      assert.is_nil(span.attributes["key1"])
 
       span:set_attribute("key1", function() end)
       assert.spy(log_spy).was_called_with(ngx.ERR, match.is_string())