Implement canister metrics

[rvanasa]

• canister_version: The current version of the canister.
• stable_memory_pages: Size of the stable memory allocated by this canister measured in 64K Wasm pages.
• request_calls: Total number of calls to the request method.
• request_cycles_charged: Total number of cycles charged in the request method.
• For each specialized Candid-RPC call (e.g. eth_get_logs, eth_get_block_by_number):
  • eth_*_calls: Total number of calls to the eth_* method.
  • eth_*_cycles_charged: Total number of cycles charged by eth_* calls.