gRPC metrics for kas

Relates to #9 (closed).

Sample metrics:

# HELP go_gc_duration_seconds A summary of the pause duration of garbage collection cycles.
# TYPE go_gc_duration_seconds summary
go_gc_duration_seconds{quantile="0"} 4.6349e-05
go_gc_duration_seconds{quantile="0.25"} 4.6349e-05
go_gc_duration_seconds{quantile="0.5"} 5.8681e-05
go_gc_duration_seconds{quantile="0.75"} 0.000176117
go_gc_duration_seconds{quantile="1"} 0.000176117
go_gc_duration_seconds_sum 0.000281147
go_gc_duration_seconds_count 3
# HELP go_goroutines Number of goroutines that currently exist.
# TYPE go_goroutines gauge
go_goroutines 33
# HELP go_info Information about the Go environment.
# TYPE go_info gauge
go_info{version="go1.15.2"} 1
# HELP go_memstats_alloc_bytes Number of bytes allocated and still in use.
# TYPE go_memstats_alloc_bytes gauge
go_memstats_alloc_bytes 6.99332e+06
# HELP go_memstats_alloc_bytes_total Total number of bytes allocated, even if freed.
# TYPE go_memstats_alloc_bytes_total counter
go_memstats_alloc_bytes_total 1.1925648e+07
# HELP go_memstats_buck_hash_sys_bytes Number of bytes used by the profiling bucket hash table.
# TYPE go_memstats_buck_hash_sys_bytes gauge
go_memstats_buck_hash_sys_bytes 1.447083e+06
# HELP go_memstats_frees_total Total number of frees.
# TYPE go_memstats_frees_total counter
go_memstats_frees_total 14066
# HELP go_memstats_gc_cpu_fraction The fraction of this program's available CPU time used by the GC since the program started.
# TYPE go_memstats_gc_cpu_fraction gauge
go_memstats_gc_cpu_fraction 3.889682570247633e-06
# HELP go_memstats_gc_sys_bytes Number of bytes used for garbage collection system metadata.
# TYPE go_memstats_gc_sys_bytes gauge
go_memstats_gc_sys_bytes 5.190944e+06
# HELP go_memstats_heap_alloc_bytes Number of heap bytes allocated and still in use.
# TYPE go_memstats_heap_alloc_bytes gauge
go_memstats_heap_alloc_bytes 6.99332e+06
# HELP go_memstats_heap_idle_bytes Number of heap bytes waiting to be used.
# TYPE go_memstats_heap_idle_bytes gauge
go_memstats_heap_idle_bytes 5.7188352e+07
# HELP go_memstats_heap_inuse_bytes Number of heap bytes that are in use.
# TYPE go_memstats_heap_inuse_bytes gauge
go_memstats_heap_inuse_bytes 8.773632e+06
# HELP go_memstats_heap_objects Number of allocated objects.
# TYPE go_memstats_heap_objects gauge
go_memstats_heap_objects 15431
# HELP go_memstats_heap_released_bytes Number of heap bytes released to OS.
# TYPE go_memstats_heap_released_bytes gauge
go_memstats_heap_released_bytes 5.6778752e+07
# HELP go_memstats_heap_sys_bytes Number of heap bytes obtained from system.
# TYPE go_memstats_heap_sys_bytes gauge
go_memstats_heap_sys_bytes 6.5961984e+07
# HELP go_memstats_last_gc_time_seconds Number of seconds since 1970 of last garbage collection.
# TYPE go_memstats_last_gc_time_seconds gauge
go_memstats_last_gc_time_seconds 1.6019522491521852e+09
# HELP go_memstats_lookups_total Total number of pointer lookups.
# TYPE go_memstats_lookups_total counter
go_memstats_lookups_total 0
# HELP go_memstats_mallocs_total Total number of mallocs.
# TYPE go_memstats_mallocs_total counter
go_memstats_mallocs_total 29497
# HELP go_memstats_mcache_inuse_bytes Number of bytes in use by mcache structures.
# TYPE go_memstats_mcache_inuse_bytes gauge
go_memstats_mcache_inuse_bytes 27776
# HELP go_memstats_mcache_sys_bytes Number of bytes used for mcache structures obtained from system.
# TYPE go_memstats_mcache_sys_bytes gauge
go_memstats_mcache_sys_bytes 32768
# HELP go_memstats_mspan_inuse_bytes Number of bytes in use by mspan structures.
# TYPE go_memstats_mspan_inuse_bytes gauge
go_memstats_mspan_inuse_bytes 173672
# HELP go_memstats_mspan_sys_bytes Number of bytes used for mspan structures obtained from system.
# TYPE go_memstats_mspan_sys_bytes gauge
go_memstats_mspan_sys_bytes 180224
# HELP go_memstats_next_gc_bytes Number of heap bytes when next garbage collection will take place.
# TYPE go_memstats_next_gc_bytes gauge
go_memstats_next_gc_bytes 1.0467584e+07
# HELP go_memstats_other_sys_bytes Number of bytes used for other system allocations.
# TYPE go_memstats_other_sys_bytes gauge
go_memstats_other_sys_bytes 2.931253e+06
# HELP go_memstats_stack_inuse_bytes Number of bytes in use by the stack allocator.
# TYPE go_memstats_stack_inuse_bytes gauge
go_memstats_stack_inuse_bytes 1.14688e+06
# HELP go_memstats_stack_sys_bytes Number of bytes obtained from system for stack allocator.
# TYPE go_memstats_stack_sys_bytes gauge
go_memstats_stack_sys_bytes 1.14688e+06
# HELP go_memstats_sys_bytes Number of bytes obtained from system.
# TYPE go_memstats_sys_bytes gauge
go_memstats_sys_bytes 7.6891136e+07
# HELP go_threads Number of OS threads created.
# TYPE go_threads gauge
go_threads 21
# HELP grpc_client_handled_total Total number of RPCs completed by the client, regardless of success or failure.
# TYPE grpc_client_handled_total counter
grpc_client_handled_total{grpc_code="OK",grpc_method="InfoRefsUploadPack",grpc_service="gitaly.SmartHTTPService",grpc_type="server_stream"} 3
grpc_client_handled_total{grpc_code="OK",grpc_method="TreeEntry",grpc_service="gitaly.CommitService",grpc_type="server_stream"} 2
# HELP grpc_client_msg_received_total Total number of RPC stream messages received by the client.
# TYPE grpc_client_msg_received_total counter
grpc_client_msg_received_total{grpc_method="InfoRefsUploadPack",grpc_service="gitaly.SmartHTTPService",grpc_type="server_stream"} 13
grpc_client_msg_received_total{grpc_method="TreeEntry",grpc_service="gitaly.CommitService",grpc_type="server_stream"} 2
# HELP grpc_client_msg_sent_total Total number of gRPC stream messages sent by the client.
# TYPE grpc_client_msg_sent_total counter
grpc_client_msg_sent_total{grpc_method="InfoRefsUploadPack",grpc_service="gitaly.SmartHTTPService",grpc_type="server_stream"} 3
grpc_client_msg_sent_total{grpc_method="TreeEntry",grpc_service="gitaly.CommitService",grpc_type="server_stream"} 2
# HELP grpc_client_started_total Total number of RPCs started on the client.
# TYPE grpc_client_started_total counter
grpc_client_started_total{grpc_method="InfoRefsUploadPack",grpc_service="gitaly.SmartHTTPService",grpc_type="server_stream"} 3
grpc_client_started_total{grpc_method="TreeEntry",grpc_service="gitaly.CommitService",grpc_type="server_stream"} 2
# HELP grpc_server_handled_total Total number of RPCs completed on the server, regardless of success or failure.
# TYPE grpc_server_handled_total counter
grpc_server_handled_total{grpc_code="Unknown",grpc_method="GetConfiguration",grpc_service="agentrpc.Kas",grpc_type="server_stream"} 9
# HELP grpc_server_msg_received_total Total number of RPC stream messages received on the server.
# TYPE grpc_server_msg_received_total counter
grpc_server_msg_received_total{grpc_method="GetConfiguration",grpc_service="agentrpc.Kas",grpc_type="server_stream"} 10
grpc_server_msg_received_total{grpc_method="GetObjectsToSynchronize",grpc_service="agentrpc.Kas",grpc_type="server_stream"} 1
# HELP grpc_server_msg_sent_total Total number of gRPC stream messages sent by the server.
# TYPE grpc_server_msg_sent_total counter
grpc_server_msg_sent_total{grpc_method="GetConfiguration",grpc_service="agentrpc.Kas",grpc_type="server_stream"} 1
grpc_server_msg_sent_total{grpc_method="GetObjectsToSynchronize",grpc_service="agentrpc.Kas",grpc_type="server_stream"} 1
# HELP grpc_server_started_total Total number of RPCs started on the server.
# TYPE grpc_server_started_total counter
grpc_server_started_total{grpc_method="GetConfiguration",grpc_service="agentrpc.Kas",grpc_type="server_stream"} 10
grpc_server_started_total{grpc_method="GetObjectsToSynchronize",grpc_service="agentrpc.Kas",grpc_type="server_stream"} 1