[v2] Add temporary expvar extension

[yurishkuro]

Which problem is this PR solving?

• Some internal components expose their state via expvar, but OTEL collector does not expose expvar handler

Description of the changes

• Create a temporary extension until upstream ticket is resolved.

How was this change tested?

$ go run ./cmd/jaeger
$ curl http://localhost:27777/
{
"badger_compaction_current_num_lsm": 0,
"badger_get_num_lsm": {},
"badger_get_num_memtable": 0,
"badger_get_num_user": 0,
...

[codecov]

Codecov Report

Attention: Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.

Project coverage is 96.78%. Comparing base (4640518) to head (5762279).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/jaeger/internal/extension/expvar/extension.go	75.00%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5986      +/-   ##
==========================================
- Coverage   96.82%   96.78%   -0.04%     
==========================================
  Files         345      348       +3     
  Lines       16523    16559      +36     
==========================================
+ Hits        15998    16027      +29     
- Misses        339      343       +4     
- Partials      186      189       +3     

Flag	Coverage Δ
badger_v1	8.02% <ø> (ø)
badger_v2	1.82% <ø> (ø)
cassandra-4.x-v1	16.61% <ø> (ø)
cassandra-4.x-v2	1.75% <ø> (ø)
cassandra-5.x-v1	16.61% <ø> (ø)
cassandra-5.x-v2	1.75% <ø> (ø)
elasticsearch-6.x-v1	18.82% <ø> (+0.03%)	⬆️
elasticsearch-7.x-v1	18.88% <ø> (+0.04%)	⬆️
elasticsearch-8.x-v1	19.08% <ø> (+0.04%)	⬆️
elasticsearch-8.x-v2	1.82% <ø> (ø)
grpc_v1	9.52% <ø> (ø)
grpc_v2	7.15% <ø> (ø)
kafka-v1	9.74% <ø> (ø)
kafka-v2	1.82% <ø> (ø)
memory_v2	1.82% <ø> (ø)
opensearch-1.x-v1	18.94% <ø> (+0.06%)	⬆️
opensearch-2.x-v1	18.93% <ø> (+0.03%)	⬆️
opensearch-2.x-v2	1.82% <ø> (+0.01%)	⬆️
tailsampling-processor	0.46% <ø> (ø)
unittests	95.27% <86.11%> (-0.04%)	⬇️

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