Add support for format expansion in the HTTP Set‑Metadata filter

*Title*: *Add support for format expansion in the HTTP Set‑Metadata filter*

## Description

[Set-Metadata filter](https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/http/set_metadata/v3/set_metadata.proto#envoy-v3-api-msg-extensions-filters-http-set-metadata-v3-metadata) today does not provide ability to use formatter while writing the metadata. For example, if I want to write the metadata with `%VIRTUAL_CLUSTER_NAME%`, it's not possible today as this would not get expanded and the metadata would just get populated as a string "%VIRTUAL_CLUSTER_NAME%".

We are using [ALS](https://www.envoyproxy.io/docs/envoy/latest/api-v3/service/accesslog/v3/als.proto#envoy-v3-api-msg-service-accesslog-v3-streamaccesslogsmessage-httpaccesslogentries) and want some of the additional fields to get propagated to the gRPC backend via metadata.

There is a way for us to do this using Lua but it'd be very nice to expand the functionality of this filter by allowing an additional `format_string` type which could be used for this purpose.