executor panic after task resume

[lance6716]

What did you do?

df.log

What did you expect to see?

No response

What did you see instead?

3m3e-server-executor-2               | panic: duplicate metrics collector registration attempted

Versions of the cluster

Dataflow Engine version (run tiflow version):

(paste Dataflow Engine version here)

[maxshuang]

May be it's better to return an registration error instead of making executor panic.
I will fix it later.

[maxshuang]

BTW, business logic should protect its own goroutine to avoid taking down the executor.

[maxshuang]

/cc @maxshuang