Allow asn header to be populate from isp_db in MaxMind geoip provider. (#39303)

Allows the asn header to be populated from the isp_db when it is
defined. If both asn_db and isp_db are defined, we fetch the asn header
from the asn DB.

Looking back at the proposed change that got approved
https://github.com/envoyproxy/envoy/pull/38714/files and reading the
documentation for the databases:
https://dev.maxmind.com/geoip/docs/databases/asn/
https://dev.maxmind.com/geoip/docs/databases/isp/

While we have clear separation of database names, there is an overlap of
fields between asn and isp dbs. Since the subscription model of Maxmind
doesn't include all dbs, if you have access only to the isp db, you can
still get the autonomous_system_number from it.

I believe this also help users that are using the isp db on the geoip
filter that haven't been able to migrate from version 1.33 but want to.

---------

Signed-off-by: Leonardo da Mata <[email protected]>
Signed-off-by: Julien Bisconti <[email protected]>

Mirrored from https://github.com/envoyproxy/envoy @ a5dbcbe81a54910c02aa76854d1396244d65d584

Author: update-envoy[bot]

envoy/extensions/geoip_providers/maxmind/v3/maxmind.proto

@@ -32,6 +32,7 @@ message MaxMindConfig {
 
   // Full file path to the Maxmind ASN database, e.g. /etc/GeoLite2-ASN.mmdb.
   // Database file is expected to have .mmdb extension.
+  // When is defined the ASN information will always be fetched from the ``asn_db``.
   string asn_db_path = 2 [(validate.rules).string = {pattern: "^$|^.*\\.mmdb$"}];
 
   // Full file path to the Maxmind anonymous IP database, e.g. /etc/GeoIP2-Anonymous-IP.mmdb.
@@ -40,6 +41,8 @@ message MaxMindConfig {
 
   // Full file path to the Maxmind ISP database, e.g. /etc/GeoLite2-ISP.mmdb.
   // Database file is expected to have .mmdb extension.
+  // If ``asn_db_path`` is not defined, ASN information will be fetched from
+  // ``isp_db`` instead.
   string isp_db_path = 5 [(validate.rules).string = {pattern: "^$|^.*\\.mmdb$"}];
 
   // Common provider configuration that specifies which geolocation headers will be populated with geolocation data.