[improve][proxy] Support disabling metrics endpoint

[michaeljmarshall]

Motivation

In certain deployments, it is preferable to be able to disable the /proxy-stats, /metrics, and /status.html endpoints.

Modifications

• Add configuration named enableProxyStatsEndpoints to the proxy configuration. When true (default), the endpoints are all exposed. When false, they are not served.
• Fix some implementation details for the proxy stats servlet configuration and the endpoints themselves

Verifying this change

This is a minor change.

Documentation

• doc-not-needed

[codecov-commenter]

Codecov Report

Merging #21031 (adc0b37) into master (0cb1c78) will increase coverage by 39.55%.
Report is 4 commits behind head on master.
The diff coverage is 55.00%.

@@              Coverage Diff              @@
##             master   #21031       +/-   ##
=============================================
+ Coverage     33.56%   73.11%   +39.55%     
- Complexity    12198    32174    +19976     
=============================================
  Files          1621     1875      +254     
  Lines        126970   139505    +12535     
  Branches      13857    15344     +1487     
=============================================
+ Hits          42618   102001    +59383     
+ Misses        78748    29436    -49312     
- Partials       5604     8068     +2464     

Flag	Coverage Δ
inttests	24.27% <30.00%> (+0.03%)	⬆️
systests	25.09% <30.00%> (?)
unittests	72.41% <55.00%> (+40.31%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...java/org/apache/pulsar/proxy/stats/ProxyStats.java	51.06% <23.80%> (+51.06%)	⬆️
...pache/pulsar/proxy/server/ProxyServiceStarter.java	65.33% <75.00%> (+23.05%)	⬆️
...apache/pulsar/proxy/server/ProxyConfiguration.java	97.56% <100.00%> (+28.70%)	⬆️
...java/org/apache/pulsar/proxy/server/WebServer.java	78.52% <100.00%> (+11.04%)	⬆️

... and 1516 files with indirect coverage changes