[enhancement](bvar) add metrics to monitor load throughput

[freemandealer]

to monitor realtime load throughput of the BE

Proposed changes

Issue Number: close #xxx

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[freemandealer]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 36.31% (8147/22437)
Line Coverage: 28.43% (65158/229196)
Region Coverage: 27.35% (33754/123403)
Branch Coverage: 24.04% (17207/71576)
Coverage Report: http://coverage.selectdb-in.cc/coverage/938fe3c628672fdb91462c08067304407003a94d_938fe3c628672fdb91462c08067304407003a94d/report/index.html

[doris-robot]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 45.72 seconds
stream load tsv: 558 seconds loaded 74807831229 Bytes, about 127 MB/s
stream load json: 20 seconds loaded 2358488459 Bytes, about 112 MB/s
stream load orc: 66 seconds loaded 1101869774 Bytes, about 15 MB/s
stream load parquet: 33 seconds loaded 861443392 Bytes, about 24 MB/s
insert into select: 29.0 seconds inserted 10000000 Rows, about 344K ops/s
storage size: 17162214284 Bytes

[dataroaring]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.