internal/lsp/debug: guard rpc stats with a Mutex

This type was previously guarded implicitly by the global exporter
mutex. With that gone, we've started seeing panics due to races in
(*rpcs).Metric.

Guard it with a mutex.

Change-Id: I2a8c670ecfbaee9422e1f1d282b1fedb86b6a5e0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/222300
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/lsp/debug/rpc.go

@@ -11,6 +11,7 @@ import (
 	"log"
 	"net/http"
 	"sort"
+	"sync"
 
 	tlm "golang.org/x/tools/internal/lsp/telemetry"
 	"golang.org/x/tools/internal/telemetry"
@@ -42,6 +43,7 @@ var rpcTmpl = template.Must(template.Must(baseTemplate.Clone()).Parse(`
 `))
 
 type rpcs struct {
+	mu       sync.Mutex
 	Inbound  []*rpcStats
 	Outbound []*rpcStats
 }
@@ -91,6 +93,8 @@ type rpcCodeBucket struct {
 }
 
 func (r *rpcs) Metric(ctx context.Context, data telemetry.MetricData) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
 	for i, group := range data.Groups() {
 		set := &r.Inbound
 		if group.Get(tlm.RPCDirection) == tlm.Outbound {

internal/telemetry/metric.go

@@ -4,7 +4,7 @@
 
 package telemetry
 
-// Data represents a single point in the time series of a metric.
+// MetricData represents a single point in the time series of a metric.
 // This provides the common interface to all metrics no matter their data
 // format.
 // To get the actual values for the metric you must type assert to a concrete