log: report error when ctx key is non-string

[ucwong]

No description provided.

[holiman]

Diff to test

diff --git a/cmd/geth/main.go b/cmd/geth/main.go
index bac74f2a6e..9c0734e4b8 100644
--- a/cmd/geth/main.go
+++ b/cmd/geth/main.go
@@ -322,6 +322,8 @@ func geth(ctx *cli.Context) error {
        }
 
        prepare(ctx)
+       log.Warn("test", 123, 123, "bar", "baz")
+       return nil
        stack, backend := makeFullNode(ctx)

Master (also this PR before my commit), note {"":123, which is wrong

[user@work go-ethereum]$ go run ./cmd/geth --verbosity 3 --log.format=json  
{"lvl":"info","msg":"Starting Geth on Ethereum mainnet...","t":"2023-05-08T09:00:00.176976284+02:00"}
{"lvl":"info","msg":"Bumping default cache on mainnet","provided":1024,"t":"2023-05-08T09:00:00.177328202+02:00","updated":4096}
{"":123,"LOG15_ERROR":"123 is not a string key","bar":"baz","lvl":"warn","msg":"test","t":"2023-05-08T09:00:00.177366763+02:00"}

This PR:

[user@work go-ethereum]$ go run ./cmd/geth --verbosity 3 --log.format=logfmt
t=2023-05-08T09:08:36+0200 lvl=info msg="Starting Geth on Ethereum mainnet..."
t=2023-05-08T09:08:36+0200 lvl=info msg="Bumping default cache on mainnet"     provided=1024 updated=4096
t=2023-05-08T09:08:36+0200 lvl=warn msg=test                                   LOG15_ERROR=int is not a string key bar=baz

[user@work go-ethereum]$ go run ./cmd/geth --verbosity 3 --log.format=json
{"lvl":"info","msg":"Starting Geth on Ethereum mainnet...","t":"2023-05-08T09:09:10.085519597+02:00"}
{"lvl":"info","msg":"Bumping default cache on mainnet","provided":1024,"t":"2023-05-08T09:09:10.086149846+02:00","updated":4096}
{"LOG15_ERROR":"int is not a string key","bar":"baz","lvl":"warn","msg":"test","t":"2023-05-08T09:09:10.086244948+02:00"}

[user@work go-ethereum]$ go run ./cmd/geth --verbosity 3 --log.format=terminal
INFO [05-08|09:09:21.642] Starting Geth on Ethereum mainnet... 
INFO [05-08|09:09:21.642] Bumping default cache on mainnet         provided=1024 updated=4096
WARN [05-08|09:09:21.642] test                                     LOG15_ERROR=int is not a string key bar=baz

[holiman]

Thanks, LGTM!