sonic-utilities: Add CLI commands to set sflow drop-monitor-limit

[sflow]

• added CLI: config sflow drop-monitor-limit 1-999 (0 to disable)
• added line to show sflow output to indicate current setting
• added test to ensure that out-of-range value is denied

The effect is to add the redis field drop_monitor_limit to the sflow configuration table in redis, as described in pull-request PR#1484 on the sonic-net/SONiC repo.

Note: hsflowd version 2.0.52 will apply this setting. Older versions will ignore it.

Example:

% sudo config sflow drop-monitor-limit 50 % show sflow

sFlow Global Information: sFlow Admin State: up sFlow Sample Direction: rx sFlow Polling Interval: default sFlow AgentID: default sFlow Drop Rate Limit: 50

Signed-off-by: Neil McKee [email protected]

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