Doc on data sharing between filters (#4816)
Doc on data sharing between filters (#4816)

Signed-off-by: Shriram Rajagopalan <shriramr@vmware.com>

Mirrored from https://github.com/envoyproxy/envoy @ d969db30237ac441e4a2d429fc4bb5f4382a0732

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api