proxy: Improve logging performance and allow separate log streaming (#421)

Replace container/ring.Ring with a custom circularBuffer that uses a
single contiguous []byte slice. This fixes the original implementation
which created 10,240 ring elements instead of 10KB of storage.

GetHistory is now 139x faster (145μs → 1μs) and uses 117x less memory
(1.2MB → 10KB). Allocations reduced from 2 to 1 per write operation.

Create a LogMonitor per proxy.Process, replacing the usage
of a shared one. The buffer in LogMonitor is lazy allocated on the first
call to Write and freed when the Process is stopped. This reduces
unnecessary memory usage when a model is not active.

The /logs/stream/{model_id} endpoint was added to stream logs from a
specific process.

README.md

@@ -203,23 +203,26 @@ As a safeguard, llama-swap also sets `X-Accel-Buffering: no` on SSE responses. H
 
 ## Monitoring Logs on the CLI
 
-```shell
+```sh
 # sends up to the last 10KB of logs
-curl http://host/logs'
+$ curl http://host/logs
 
 # streams combined logs
-curl -Ns 'http://host/logs/stream'
+curl -Ns http://host/logs/stream
 
-# just llama-swap's logs
-curl -Ns 'http://host/logs/stream/proxy'
+# stream llama-swap's proxy status logs
+curl -Ns http://host/logs/stream/proxy
 
-# just upstream's logs
-curl -Ns 'http://host/logs/stream/upstream'
+# stream logs from upstream processes that llama-swap loads
+curl -Ns http://host/logs/stream/upstream
+
+# stream logs only from a specific model
+curl -Ns http://host/logs/stream/{model_id}
 
 # stream and filter logs with linux pipes
 curl -Ns http://host/logs/stream | grep 'eval time'
 
-# skips history and just streams new log entries
+# appending ?no-history will disable sending buffered history first
 curl -Ns 'http://host/logs/stream?no-history'
 ```