prometheus metrics for external stream sink

[qijun-niu-timeplus]

PR checklist:

• Did you run ClangFormat ?
• Did you separate headers to a different section in existing community code base ?
• Did you surround proton: starts/ends for new code in existing community code base ?

Please write user-readable short description of the changes:

how to check:

1. create a topic test on redpanda/kafka

2. proton client

create external stream test(i int, j int) settings type='kafka', brokers='redpanda:9092', topic='test', data_format='JSONEachRow';
insert into test(i, j) values(1, 2)(3, 4)(5, 6);
select * from test settings seek_to='earliest'

3. query prometheus metrics

curl http://localhost:9363/metrics

response:

...
# TYPE ProtonExternalStream_ReadBytes counter
ProtonExternalStream_ReadBytes{database="default", name="test"} 42
# TYPE ProtonExternalStream_ReadCounts counter
ProtonExternalStream_ReadCounts{database="default", name="test"} 3
# TYPE ProtonExternalStream_ReadFailed counter
ProtonExternalStream_ReadFailed{database="default", name="test"} 0
# TYPE ProtonExternalStream_WriteBytes counter
ProtonExternalStream_WriteBytes{database="default", name="test"} 42
# TYPE ProtonExternalStream_WriteCounts counter
ProtonExternalStream_WriteCounts{database="default", name="test"} 3
# TYPE ProtonExternalStream_WriteFailed counter
ProtonExternalStream_WriteFailed{database="default", name="test"} 0

[yokofly]

overall is good,

we can consider to use relaxed for this pure atomic counter. counter is not related to happen before. this is sufficient.

No ordering constraints, only atomic operations.

but we can create a new ticket for statistically the metrics.
https://stackoverflow.com/a/27716387
https://en.cppreference.com/w/cpp/atomic/memory_order#Relaxed_ordering

[yokofly]

create a new ticket here #488