Fix bug that WebSocket proxy returns empty metrics

[massakam]

Motivation

WebSocket proxy provides an endpoint for obtaining metrics.

/admin/proxy-stats/metrics

However, I realized that the endpoint almost always returns empty JSON [].
There are two causes for this.

First, there is a possibility that NullPointerException occurs in this line:
https://github.com/apache/incubator-pulsar/blob/5fc4d536d76b1d73c6be7b9238bf14b210ee095f/pulsar-websocket/src/main/java/org/apache/pulsar/websocket/stats/ProxyStats.java#L119
publishMsgLatency is not initialized unless at least one producer is connected.
https://github.com/apache/incubator-pulsar/blob/5fc4d536d76b1d73c6be7b9238bf14b210ee095f/pulsar-websocket/src/main/java/org/apache/pulsar/websocket/stats/ProxyStats.java#L72-L74

Seccond, tempMetricsCollection is not an local variable but an instance variable:
https://github.com/apache/incubator-pulsar/blob/5fc4d536d76b1d73c6be7b9238bf14b210ee095f/pulsar-websocket/src/main/java/org/apache/pulsar/websocket/stats/ProxyStats.java#L44
As a result, the three variables tempMetricsCollection, tempRef and metricsCollection always refer to the same instance, and metricsCollection is cleared each time after the second and subsequent executions of generate().
https://github.com/apache/incubator-pulsar/blob/5fc4d536d76b1d73c6be7b9238bf14b210ee095f/pulsar-websocket/src/main/java/org/apache/pulsar/websocket/stats/ProxyStats.java#L97-L99

Modifications

• Initialize publishMsgLatency in the constructor of ProxyNamespaceStats
• Declare tempMetricsCollection as local variable

Result

WebSocket proxy no longer returns empty metrics.

[merlimat]

👍