Prevent sensitive request headers from being logged in debug level

*Description*:
Hey all, first just some quick background before getting into the issue we're experiencing. We wrote a small service that is called by the `ext_authz` filter to perform an OIDC Authorization Flow, and verify existing JWTs stored as cookies in request headers.

The JWT cookie contains sensitive information and therefore shouldn't be logged, however we noticed that when envoy is run with `-l debug`, various components will log the full request headers including the JWT, e.g.:

```
envoy_1 | [2020-01-10 16:29:21.082][14][debug][http] [source/common/http/conn_manager_impl.cc:246] [C5] new stream
envoy_1 | [2020-01-10 16:29:21.082][14][debug][http] [source/common/http/conn_manager_impl.cc:619] [C5][S6425470214323392008] request headers complete (end_stream=true):
envoy_1 | ':authority', 'localhost:9002'
envoy_1 | ':path', '/say'
envoy_1 | ':method', 'GET'
envoy_1 | 'user-agent', 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:71.0) Gecko/20100101 Firefox/71.0'
envoy_1 | 'accept', 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'
envoy_1 | 'accept-language', 'en-US,en;q=0.5'
envoy_1 | 'accept-encoding', 'gzip, deflate'
envoy_1 | 'dnt', '1'
envoy_1 | 'connection', 'keep-alive'
envoy_1 | 'cookie', 'oidc-service-token=supersecrettoken'
envoy_1 | 'upgrade-insecure-requests', '1'
envoy_1 | 'x-svc-cluster', 'sayer'
```
and

```
envoy_1 | [2020-01-10 16:29:21.083][14][debug][router] [source/common/router/router.cc:514] [C5][S6425470214323392008] router decoding headers:
envoy_1 | ':authority', 'localhost:9002'
envoy_1 | ':path', '/say'
envoy_1 | ':method', 'GET'
envoy_1 | ':scheme', 'http'
envoy_1 | 'user-agent', 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:71.0) Gecko/20100101 Firefox/71.0'
envoy_1 | 'accept', 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'
envoy_1 | 'accept-language', 'en-US,en;q=0.5'
envoy_1 | 'accept-encoding', 'gzip, deflate'
envoy_1 | 'dnt', '1'
envoy_1 | 'cookie', 'oidc-service-token=supersecrettoken'
envoy_1 | 'upgrade-insecure-requests', '1'
envoy_1 | 'x-svc-cluster', 'sayer'
envoy_1 | 'x-forwarded-for', '172.22.0.1'
envoy_1 | 'x-forwarded-proto', 'http'
envoy_1 | 'x-envoy-internal', 'true'
envoy_1 | 'x-request-id', '635a3677-6c36-404e-9421-8c7a1f37304e'
envoy_1 | 'x-envoy-expected-rq-timeout-ms', '20000'
```

Normally we don't run our production envoy's with debug level, however there are instances where we will temporarily set the level to debug when troubleshooting issues etc.

I'm wondering if there is some way to exclude particular headers from being logged in debug level? The only documentation I can seem to find for configuring logging is for access logs, not envoy's application logs.

Thanks!