route: replace ENVOY_BUG to ENVOY_LOG

[wbpcode]

Commit Message: route: replace ENVOY_BUG to ENVOY_LOG
Additional Description:

To fix #28626.

To avoid holding the whole route table in a long time and accelarate the release of memory, we forbidden route refreshment or clear after the response headers are sent in the #26045. And an ENVOY_BUG was introduced to ensure that.

This work well in most case because in most case we never refresh route at response phase (and it make no sense). But there are some corner cases will result the ENVOY_BUG be triggerd unexpectedly.
There are two mainly corner cases:

1. The local response is sent before the initial route is set. This may occur because the request lost some necessary headers.
2. Call the sendLocalReply after the route cache is cleared in the request/decoder filter chain.
   In both cases above, if a response/encoder filter try to access route by route() method, the ENVOY_BUG may be triggered.

To resolve the problem, this PR do two things:

1. Ensure the route is set before send local response. This means even the initial route is not set or the route is cleared by filters, we will set the route before the response/encoder filter chain processing starts. (We needn't do this for normal response because router filter will do this. And note the route still may be cleared by response/encode filters, but this case is more like filter bug and should be handled by the filter developers.)
2. Replace ENVOY_BUG to ENVOY_LOG. Seems ENVOY_BUG is too strict for this scenario. I think warning log should be enough. And even there are some uncovered corner cases, warning log would more friendly than ENVOY_BUG.

Risk Level: low.
Testing: n/a.
Docs Changes: n/a.
Release Notes: n/a.
Platform Specific Features: n/a.

[wbpcode]

not sure is this a good solution. but at least better than do nothing. 😞

[mattklein123]

So this is a NOP if the route is already cached? Just making sure we aren't doing extra work on this path for the common case?

[wbpcode]

Yeah, if a route is already cached, then the route() will return directly and this calling will do nothing.

Router::RouteConstSharedPtr
ConnectionManagerImpl::ActiveStream::route(const Router::RouteCallback& cb) {
  if (cached_route_.has_value()) {
    return cached_route_.value();
  }
  refreshCachedRoute(cb);
  return cached_route_.value();
}