support monitoring memcached metrics and add a help doc

[ZY945]

What's changed?

support monitoring memcached metrics and add a help doc

Checklist

• I hereby agree to the terms of the HertzBeat CLA
• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[tomsun28]

LGTM! 👍👍
and please fix the pmd error

[INFO] --- maven-pmd-plugin:3.12.0:check (validate) @ hertzbeat-collector ---
[INFO] PMD Failure: org.dromara.hertzbeat.collector.collect.memcached.MemcachedCollectImpl:61 Rule:CollectionInitShouldAssignCapacityRule Priority:3 【HashMap】初始化时，尽量指定初始值大小.
[INFO] PMD Failure: org.dromara.hertzbeat.collector.collect.memcached.MemcachedCollectImpl:115 Rule:UndefineMagicConstantRule Priority:3 魔法值【"END"】.
[INFO] PMD Failure: org.dromara.hertzbeat.collector.collect.memcached.MemcachedCollectImpl:129 Rule:UndefineMagicConstantRule Priority:3 魔法值【"END"】.

[ZY945]

LGTM! 👍👍 and please fix the pmd error

[INFO] --- maven-pmd-plugin:3.12.0:check (validate) @ hertzbeat-collector ---
[INFO] PMD Failure: org.dromara.hertzbeat.collector.collect.memcached.MemcachedCollectImpl:61 Rule:CollectionInitShouldAssignCapacityRule Priority:3 【HashMap】初始化时，尽量指定初始值大小.
[INFO] PMD Failure: org.dromara.hertzbeat.collector.collect.memcached.MemcachedCollectImpl:115 Rule:UndefineMagicConstantRule Priority:3 魔法值【"END"】.
[INFO] PMD Failure: org.dromara.hertzbeat.collector.collect.memcached.MemcachedCollectImpl:129 Rule:UndefineMagicConstantRule Priority:3 魔法值【"END"】.

Is it possible to modify the magic number through static variables

[tomsun28]

Is it possible to modify the magic number through static variables

of course