metrics

[turuslan]

• instance count and list annotations for metrics.
• count libp2p instances: tcp/noise/yamuxed connection, upgrader session, yamux stream, gossip stream and peer context.
• metrics test.
• fix unused warning (yamuxed_connection.cpp).

[igor-egorov]

Object creation or destruction locks and unlocks a mutex. That looks to be a too heavy operation

[turuslan]

count()'s count is static local variable, so this lambda with mutex locking is called only once, to insert key-counter pair into map.
Generally LIBP2P_METRICS_INSTANCES_COUNT constructor/destructor is just checking "static guard variable" (was static variable initialized) followed by atomic increment/decrement.

LIBP2P_METRICS_INSTANCES_LIST is heavier and always locks mutex, because it inserts/erases list. Developer chooses whether to keep list of all type instances or not (or just count with LIBP2P_METRICS_INSTANCES_COUNT).

[turuslan]

disabled by default via cmake option

[art-gor]

Better to have this feature disabled by default in build time, and to be enabled explicitly by CMake variable and preprocessor,
cmake ... -DMETRICS_ENABLED=On
...
if(METRICS_ENABLED)
add_compile_definitions("METRICS_ENABLED")
... etc

[art-gor]

commented earlier about making this feature a non-default option