[WIP] AO-14906: remove the old udp reporter

[jiwen624]

WIP.

Currently, there are a few fields, e.g., the custom metrics, are attached to the gRPC reporter which makes it not just a transport layer as indicated by its name. On the other hand, the UDP reporter is just a thin layer reporting events/metrics via UDP.

In addition, a few global variables exist in the package to do the protocol-independent tasks. For example, the global settings and configurations. These global variables make testing a tough job. What's more, in order to add a new feature we may have to either implement it in each reporter or use another global variable.

For ease of testing, we're trying to eliminate the global variables by attaching the settings and configurations to the reporter and make the (unified) reporter protocol-agnostic. The transport layer (either gRPC or UDP or something else) could be a field of the reporter which only does the final data transportation work.

The first step to achieve the above goal is to remove the old UDP reporter. It will be added back (in a different way) in case it's needed in the future.

[codecov-io]

Codecov Report

Merging #135 into master will increase coverage by 0.89%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage   81.91%   82.81%   +0.89%     
==========================================
  Files          37       36       -1     
  Lines        4590     4538      -52     
==========================================
- Hits         3760     3758       -2     
+ Misses        705      656      -49     
+ Partials      125      124       -1

Impacted Files	Coverage Δ
v1/ao/internal/config/config.go	83.37% <ø> (-0.19%)	⬇️
v1/ao/internal/config/wrappers.go	33.33% <ø> (ø)	⬆️
v1/ao/internal/reporter/reporter.go	81.9% <100%> (+1.53%)	⬆️
v1/ao/internal/config/validators.go	96% <100%> (ø)	⬆️
v1/ao/internal/reporter/reporter_grpc.go	69.67% <0%> (+0.29%)	⬆️

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