benchmark: refactor watch benchmark

[heyitsanthony]

Hangs at/above 100 streams due to a hardcoded constant defaultMaxStreamsClient in grpc (having multiple grpc connections doesn't seem to make a difference)

Supersedes #7922, #8057

[heyitsanthony]

OK, I was setting --conns but not --clients, so it can get above 100. Will add a warning in the benchmark if it's exceeding the limit.

[codecov-io]

Codecov Report

Merging #8069 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8069      +/-   ##
==========================================
- Coverage   76.66%   76.65%   -0.01%     
==========================================
  Files         341      341              
  Lines       26511    26511              
==========================================
- Hits        20324    20322       -2     
- Misses       4741     4743       +2     
  Partials     1446     1446

Impacted Files	Coverage Δ
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
clientv3/namespace/watch.go	93.93% <0%> (-6.07%)	⬇️
pkg/adt/interval_tree.go	89.63% <0%> (-3.66%)	⬇️
etcdserver/api/v3election/election.go	64.7% <0%> (-2.95%)	⬇️
raft/node.go	90.17% <0%> (-2.24%)	⬇️
rafthttp/msgappv2_codec.go	71.3% <0%> (-1.74%)	⬇️
rafthttp/peer.go	90.22% <0%> (-1.51%)	⬇️
etcdserver/server.go	80.74% <0%> (-0.24%)	⬇️
clientv3/watch.go	96.25% <0%> (+0.75%)	⬆️
mvcc/watchable_store.go	88.75% <0%> (+0.77%)	⬆️
... and 8 more

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[gyuho]

lgtm. tried it and works well.