support streamnode

[suchenglong]

Description

Content1: Refactor - refactor metric framework to support additional prefixes

In MetricConfigDescriptor, the node-type property prefixes "cn_" and "dn_" were previously inlined as string
literals in three call sites: the two boolean isConfigNode overloads of loadProps/loadHotProps (which forward to
the String prefix overloads via a ternary), and the if (prefix.equals(...)) guard inside generateFromProperties
that limits the internal-reporter config to DataNode.

These literals are now declared as two private static final String constants near the top of the class:

private static final String CONFIG_NODE_PREFIX = "cn_";
private static final String DATA_NODE_PREFIX = "dn_";

The three use sites now reference the constants instead of the literals. No other strings in the file (property keys
such as metric_reporter_list, default values, comments mentioning "cn_"/"dn_"/"sn_" in javadoc) were touched,
per the scope of the change.

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This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write