feat: enable TcpClient metrics

[rxdcxdrnine]

Motivation:
cannot check if LoopResources event loop has pending tasks with metrics.

Modification:
add TcpClient.metrics(true) with if statements, check if micrometer is available.

Result:
expose the metrics of LoopResources including reactor_netty_event_loop_pending_tasks.
The following is a list of metrics exposed when the commit was applied (with default TcpResources)

reactor_netty_eventloop_pending_tasks{name="reactor-tcp-nio-4"} 0.0
reactor_netty_eventloop_pending_tasks{name="reactor-tcp-nio-5"} 0.0
reactor_netty_eventloop_pending_tasks{name="reactor-tcp-nio-6"} 0.0
reactor_netty_eventloop_pending_tasks{name="reactor-tcp-nio-7"} 0.0
reactor_netty_eventloop_pending_tasks{name="reactor-tcp-nio-8"} 0.0
reactor_netty_tcp_client_address_resolver_seconds_count{remote_address="localhost:64921",status="SUCCESS"} 100
reactor_netty_tcp_client_address_resolver_seconds_sum{remote_address="localhost:64921",status="SUCCESS"} 1.095592293
reactor_netty_tcp_client_address_resolver_seconds_max{remote_address="localhost:64921",status="SUCCESS"} 0.01396475
reactor_netty_tcp_client_connect_time_seconds_count{proxy_address="na",remote_address="localhost:64921",status="SUCCESS"} 100
reactor_netty_tcp_client_connect_time_seconds_sum{proxy_address="na",remote_address="localhost:64921",status="SUCCESS"} 3.49526084
reactor_netty_tcp_client_connect_time_seconds_max{proxy_address="na",remote_address="localhost:64921",status="SUCCESS"} 0.039677708
reactor_netty_tcp_client_data_received_bytes_count{proxy_address="na",remote_address="localhost:64921",uri="tcp"} 814
reactor_netty_tcp_client_data_received_bytes_sum{proxy_address="na",remote_address="localhost:64921",uri="tcp"} 325900.0
reactor_netty_tcp_client_data_received_bytes_max{proxy_address="na",remote_address="localhost:64921",uri="tcp"} 2048.0
reactor_netty_tcp_client_data_sent_bytes_count{proxy_address="na",remote_address="localhost:64921",uri="tcp"} 1200
reactor_netty_tcp_client_data_sent_bytes_sum{proxy_address="na",remote_address="localhost:64921",uri="tcp"} 108700.0

[rxdcxdrnine]

@jchrys
I added property named metrics in MysqlConnectionConfiguration.

• define property as primitive type for setting the default value with false.
• if you call Builder.metrics() without micrometer-core dependency, throw IllegalArgumentException for fast fail.

Thanks!

[jchrys]

@rxdcxdrnine Thanks a lot!

[jchrys]

@rxdcxdrnine To maintain a clean and concise commit history, could you please squash your commits into a single one?

[rxdcxdrnine]

@jchrys
I squashed commits and force pushed to this branch.

[jchrys]

LGTM