fix(server-timings): lock header while inspecting/modifying Extra

[rvagg]

Reported from Saturn:

fatal error: concurrent map writes

goroutine 8048914 [running]:
github.com/filecoin-project/lassie/pkg/server/http.ipfsHandler.func1.3({0x1831098, 0xc0268c0cf0})
        /github/workspace/pkg/server/http/ipfs.go:250 +0x346
github.com/filecoin-project/lassie/pkg/retriever.makeOnRetrievalEvent.func1({0x1831098?, 0xc0268c0cf0?})
        /github/workspace/pkg/retriever/retriever.go:226 +0x3ba
github.com/filecoin-project/lassie/pkg/retriever.collectResults({0x18252b0, 0xc001ee9600}, 0xc01c73f4c0, 0xc01b61dc20)
        /github/workspace/pkg/retriever/graphsyncretriever.go:242 +0x1bf
github.com/filecoin-project/lassie/pkg/retriever.(*graphsyncRetrieval).RetrieveFromAsyncCandidates(0xc0147c0500, 0xc01b61dec0)
        /github/workspace/pkg/retriever/graphsyncretriever.go:215 +0x458
github.com/filecoin-project/lassie/pkg/types.AsyncRetrievalTask.Run(...)
        /github/workspace/pkg/types/types.go:131
github.com/filecoin-project/lassie/pkg/retriever/coordinators.Race.func1.1.1()
        /github/workspace/pkg/retriever/coordinators/race.go:23 +0x90
created by github.com/filecoin-project/lassie/pkg/retriever/coordinators.Race.func1.1
        /github/workspace/pkg/retriever/coordinators/race.go:21 +0x105

Header has a lock: https://pkg.go.dev/github.com/mitchellh/[email protected]#Header and it's used for the header.NewMetric call just below this section, so this is a double-lock unfortunately.

Longer-term solution might be to go back to the state we had originally where this callback is only called on the goroutine of the caller, so there's no concurrent executions at all. I think we changed that when we added bitswap retriever and added the complexity of scheduling and racing.

[codecov-commenter]

Codecov Report

Merging #220 (d98ab7b) into main (a2df0b9) will increase coverage by 0.15%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
+ Coverage   70.62%   70.78%   +0.15%     
==========================================
  Files          62       62              
  Lines        5270     5274       +4     
==========================================
+ Hits         3722     3733      +11     
+ Misses       1350     1344       -6     
+ Partials      198      197       -1     

Impacted Files	Coverage Δ
pkg/server/http/ipfs.go	59.63% <100.00%> (+0.75%)	⬆️

... and 5 files with indirect coverage changes