Support prefix option for Logback integration

Support prefix option <exportPrefix> for Logback integration to specify the prefix of exported MDC properties.

For example, if this xml is loaded

  <appender name="RCEA" class="com.linecorp.armeria.common.logback.RequestContextExportingAppender">
    <!-- specify the prefix of exported MDC properties -->
    <exportPrefix>armeria</exportPrefix>
    <export>req.*</export>
    <export>res.*</export>
    ...
  </appender>

then RequestContextExportingAppender exports armeria.req.path, armeria.req.service_name, armeria.res.status_code, etc.

[trustin]

Another thing to consider is that we might add support for other logging frameworks in the future, e.g. log4j2. We might want to keep the same set of XML tags if possible for consistent user experience.

[trustin]

How about <exportGroup> ?

<!-- If not wrapped with exportGroup, all directives go to the 'default export group'. -->
<prefix>...</prefix> <!-- This sets the prefix for all exports in the default export group. -->
<export>...</export>
<exports>...</exports>

<!-- exportGroup creates a new export group. -->
<exportGroup>
  <prefix>...</prefix> <!-- This sets the prefix for the exports in its group only. -->
  <export>...</export>
  <exports>...</exports>
</exportGroup>

<exportGroup>
  <export>...</export>
  <prefix>...</prefix> <!-- A user should be able to specify a prefix later -->
</exportGroup>

<exportGroup>
  <!-- No prefix tag means 'inherit from the default export group. -->
  <export>...</export>
</exportGroup>

One thing to think about is whether we need to allow an empty prefix, i.e. <prefix />. It looks somewhat confusing, so it’s probably better throwing an exception? Users can still achieve what they want with proper grouping of exports:

<exportGroup>
  <export>...</export>
</exportGroup>
<exportGroup>
  <prefix>...</prefix>
  <export>...</export>
</exportGroup>

We might also want to an additional tag that excludes some exports:

<!-- Add armeria prefix for all request properties except req.foo. -->
<export>req.foo</export>
<exportGroup>
  <prefix>armeria</prefix>
  <exports>req.*</exports>
  <exclusion>^req\.foo$</exclusion> <!-- regex -->
</exportGroup>

Makes sense?