Feature request for modifying how zipkin tracer acts upon receiving bad headers

Description:

Send non hex characters as request header x-b3-traceid or x-b3-spanid, or send headers that are not of 64/128bit hex length.
The tracer operates correctly and securely in the sense that it was not a sampled trace and was not submitted to zipkin collector.

However, this unverified data can then be sent to upstream, to access logs, added in headers_to_add, etc, with a small possible risk of introducing malicious input handled somewhere that was expecting a mandated format.
Request to validate the traceid and span ids are of appropriate format, otherwise reject the supplied trace and **start a new trace** (just like what happens when trace is missing entirely) so that systems expect to always have valid headers with this config. An alternative is to strip all the headers. I believe that starting a new trace is the behavior of spring-cloud-sleuth.

Config:

```
+ tracing:
+ provider:
+ name: envoy.tracers.zipkin
+ typed_config:
+ "@type": type.googleapis.com/envoy.config.trace.v3.ZipkinConfig
+ trace_id_128bit: true
+ collector_cluster: remote_zipkin
+ collector_endpoint: "/api/v2/spans"
+ collector_endpoint_version: HTTP_JSON
```

Logs:
There are no logs at debug level when this rejects the trace. Possibly could be logged or gauged.

Relevant:
The below curl output demonstrates echoing back the input – notice it is not sampled with x-b3-sampled: 1
```
curl -k -v https://localhost/echo -H "x-b3-traceid: 273d1yyyyyyyyyyyyyyy <>{} @#$%^&*()" -H "x-b3-spanid: c115c94374c93923"
> GET /echo HTTP/1.1
> Host: localhost
> User-Agent: curl/7.54.0
> Accept: */*
> x-b3-traceid: 273d1yyyyyyyyyyyyyyy <>{} @#$%^&*()
> x-b3-spanid: c115c94374c93923
>
< HTTP/1.1 200 OK
< date: Mon Aug 17 2020 13:23:46 GMT-0700 (Pacific Daylight Time)
< content-type: text/plain
< access-control-allow-origin: *
< x-envoy-upstream-service-time: 1
< x-response-traceid: 273d1yyyyyyyyyyyyyyy <>{} @#$%^&*() ****<- added via response_headers_to_add
< server: envoy
< transfer-encoding: chunked
<
GET /echo HTTP/1.1
host: localhost
user-agent: curl/7.54.0
accept: */*
x-b3-traceid: 273d1yyyyyyyyyyyyyyy <>{} @#$%^&*()
x-b3-spanid: c115c94374c93923
x-forwarded-proto: https
x-envoy-expected-rq-timeout-ms: 15000
content-length: 0
```