Add optional structured logs of RPC related events

[rem1-dev]

Adding RPC related event logs that will be used in Blockstream Explorer project. The logs are structured as json object. This optional logging may be used in an enterprise/internal scenario for billing purposes. By default it's turned off.

The logs are controlled with a new --rpc-logging command line option. Command line option possible values, and example structured logs they produce, are as following:

• --rpc-logging full results in logs:

ELECTRUM-RPC-LOGGER: {"event":"connection established","source":{"ip":"127.0.0.1","port":59877}}
ELECTRUM-RPC-LOGGER: {"event":"rpc request","id":18,"method":"blockchain.scripthash.get_history","params":["6b63eef944d982701eb2d0dbb8ee900f42d8e79fe3d1ea473602c0edc87c34f6"],"source":{"ip":"127.0.0.1","port":59877}}
ELECTRUM-RPC-LOGGER: {"duration_µs":114,"event":"rpc response","id":18,"method":"blockchain.scripthash.get_history","payload_size":37,"source":{"ip":"127.0.0.1","port":59877}}
ELECTRUM-RPC-LOGGER: {"event":"connection closed","source":{"ip":"127.0.0.1","port":59877}}

• --rpc-logging no-params excludes RPC request params to be logged, results in logs like:

ELECTRUM-RPC-LOGGER: {"event":"connection established","source":{"ip":"127.0.0.1","port":59886}}
ELECTRUM-RPC-LOGGER: {"event":"rpc request","id":18,"method":"blockchain.scripthash.get_history","params":null,"source":{"ip":"127.0.0.1","port":59886}}
ELECTRUM-RPC-LOGGER: {"duration_µs":78,"event":"rpc response","id":18,"method":"blockchain.scripthash.get_history","payload_size":37,"source":{"ip":"127.0.0.1","port":59886}}
ELECTRUM-RPC-LOGGER: {"event":"connection closed","source":{"ip":"127.0.0.1","port":59886}}```

[shesek]

I was thinking that it might be useful to log two extra things - the jsonrpc id field so that the rpc request and rpc response entries can be matched to each-other, and the time it took to process the request.

I'm not sure whether these are necessary or not. If not, we can go ahead and merge.

[rem1-dev]

Yes, it's good idea to add id into req and resp logs. Did you mean id that we can get from cmd variable: cmd.get("id")? I added that.

[shesek]

I made some minor formatting/refactoring suggestions and added the time duration for processing requests in a branch. What do you think?

[rem1-dev]

It looks good to me. I also updated PR description to be more verbose on why we're adding it.

[shesek]

Great :) Can you apply the changes on your branch and rebase?

[rem1-dev]

I applied your changes + I changed duration timestamp field to microseconds because when I tested it I never got duration > 1s and that field was always 0. Re rebase: not sure what should I rebase on. Branch is up-to date with new-index

[shesek]

Thanks!

I accidentally merged without squashing, looking into fixing this