Send User-Agent header with commit hash (#56)

This PR adds a User-Agent header on the format `node-indexer/<hash>`. We
try follow the convention described [on
Wikipedia](https://en.wikipedia.org/wiki/User-Agent_header#:~:text=Mozilla/%5Bversion%5D%20(%5Bsystem%20and%20browser%20information%5D)%20%5Bplatform%5D%20(%5Bplatform%20details%5D)%20%5Bextensions%5D).

This will be very useful when debugging!

We inject the Git hash using option 1 from Adam's excellent resource
https://developers.redhat.com/articles/2022/11/14/3-ways-embed-commit-hash-go-programs#1__using__ldflags.
Option 3 doesn't work because even when building with `-buildvcs`, a `go
build <file>` doesn't embed the VCS info, as tracked in this open issue:
golang/go#51279 (comment)

Have verified this locally using Docker build.

.gitignore

@@ -20,3 +20,6 @@
 # Go workspace file
 go.work
 go.work.sum
+
+# Binary
+indexer

Makefile

@@ -19,7 +19,7 @@ bin/gofumpt: bin
 	GOBIN=$(PWD)/bin go install mvdan.cc/gofumpt@v0.6.0
 
 build: cmd/main.go
-	CGO_ENABLED=0 go build -o indexer cmd/main.go
+	CGO_ENABLED=0 go build -ldflags="-X github.com/duneanalytics/blockchain-ingester/client/duneapi.commitHash=$(shell git rev-parse --short HEAD)" -o indexer cmd/main.go
 
 lint: bin/golangci-lint bin/gofumpt
 	go fmt ./...

client/duneapi/client.go

@@ -138,6 +138,12 @@ func (c *client) buildRequest(payloads []models.RPCBlock, buffer *bytes.Buffer)
 	return request, nil
 }
 
+// We inject the commit hash here at build time, using the -X linker flag, so we can use it in the User-Agent header
+var (
+	commitHash string
+	userAgent  = fmt.Sprintf("node-indexer/%s", commitHash)
+)
+
 func (c *client) sendRequest(ctx context.Context, request BlockchainIngestRequest) error {
 	start := time.Now()
 	var err error
@@ -172,6 +178,7 @@ func (c *client) sendRequest(ctx context.Context, request BlockchainIngestReques
 		req.Header.Set("Content-Encoding", request.ContentEncoding)
 	}
 	req.Header.Set("Content-Type", "application/x-ndjson")
+	req.Header.Set("User-Agent", userAgent)
 	req.Header.Set("x-idempotency-key", request.IdempotencyKey)
 	req.Header.Set("x-dune-evm-stack", request.EVMStack)
 	req.Header.Set("x-dune-api-key", c.cfg.APIKey)