[chassis-packet][SNMP] Enable SNMP dynamic frequency on packet chassis (#21430)

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#### Why I did it
It's one part of the fixes of #21314
SNMP walker request will always timeout when 100% CPU utilization.
##### Work item tracking
- Microsoft ADO **30112399**:

#### How I did it
Enable SNMP dynamic frequency on packet chassis.
#### How to verify it
snmp/test_snmp_cpu.py(https://github.com/sonic-net/sonic-mgmt/blob/master/tests/snmp/test_snmp_cpu.py) tests the scenario.

And the test case passes in my local run.
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#### Description for the changelog
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pull request for inclusion in the changelog:
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For issue:
#21314

Change of SNMP-AgentX sonic-net/sonic-snmpagent#345 will fix the issue.
But we want to limit the scope of the change, since we only see this issue on packet-chassis and old devices with low performance CPUs.

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Author: mssonicbld

dockers/docker-snmp/Dockerfile.j2

@@ -66,14 +66,15 @@ RUN apt-get -y purge     \
     find / | grep -E "__pycache__" | xargs rm -rf && \
     rm -rf /debs /python-wheels ~/.cache
 
+COPY ["docker-snmp-init.sh", "/usr/bin/"]
 COPY ["start.sh", "/usr/bin/"]
 COPY ["snmp_yml_to_configdb.py", "/usr/bin/"]
-COPY ["supervisord.conf", "/etc/supervisor/conf.d/"]
 COPY ["*.j2", "/usr/share/sonic/templates/"]
 COPY ["files/supervisor-proc-exit-listener", "/usr/bin"]
 COPY ["critical_processes", "/etc/supervisor"]
 
 # Although exposing ports is not needed for host net mode, keep it for possible bridge mode
 EXPOSE 161/udp 162/udp
 
-ENTRYPOINT ["/usr/local/bin/supervisord"]
+RUN chmod +x /usr/bin/docker-snmp-init.sh
+ENTRYPOINT ["/usr/bin/docker-snmp-init.sh"]

dockers/docker-snmp/docker-snmp-init.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+#Generate supervisord.conf based on device metadata
+mkdir -p /etc/supervisor/conf.d/
+sonic-cfggen -d -t /usr/share/sonic/templates/supervisord.conf.j2 > /etc/supervisor/conf.d/supervisord.conf
+exec /usr/local/bin/supervisord

dockers/docker-snmp/supervisord.conf dockers/docker-snmp/supervisord.conf.j2

@@ -50,7 +50,11 @@ dependent_startup=true
 dependent_startup_wait_for=start:exited
 
 [program:snmp-subagent]
+{% if DEVICE_METADATA['localhost']['switch_type'] == 'chassis-packet' %}
+command=/usr/bin/env python3 -m sonic_ax_impl --enable_dynamic_frequency
+{% else %}
 command=/usr/bin/env python3 -m sonic_ax_impl
+{% endif %}
 priority=4
 autostart=false
 autorestart=false