runtime: flexible layering configuration. (#7032)
runtime: flexible layering configuration. (#7032)

To support TDS and to provide operator flexibility in how it relates to
existing runtime layering, this PR introduces support for explicit
control over layering. This eliminates the distinguished role of
subdirectory and override_subdirectory, for example, in favor of
arbitrary disk overlay layers.

This PR deprecates the existing runtime configuration in favor of the
new layered variant.

As a bonus, the PR fixes some of the runtime stats and related docs.

Risk level: Low
Testing: various unit tests (including server_test) additions.

Part of #6708.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 4dafba65baaf9769723f895761268eed31af629b

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